ENH: Chain exceptions to give better error messages for invalid PEP3118 format strings

Sample console output:
```
In [1]: from ctypes import c_wchar

In [2]: x = c_wchar * 3

In [3]: np.array(x())
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~\Repos\numeric-python\numpy\build\testenv\Lib\site-packages\numpy\core\_internal.py in _dtype_from_pep3118(spec)
    486
--> 487     dtype, align = __dtype_from_pep3118(stream, is_subdtype=False)
    488     return dtype

~\Repos\numeric-python\numpy\build\testenv\Lib\site-packages\numpy\core\_internal.py in __dtype_from_pep3118(stream, is_subdtype)
    558         else:
--> 559             raise ValueError("Unknown PEP 3118 data type specifier %r" % stream.s)
    560

ValueError: Unknown PEP 3118 data type specifier 'u'

The above exception was the direct cause of the following exception:

ValueError                                Traceback (most recent call last)
<ipython-input-3-f8a17df3aa24> in <module>()
----> 1 np.array(x())

ValueError: '<u' is not a valid PEP 3118 buffer format string
```

4 files changed, 49 insertions, 33 deletions

diff --git a/numpy/core/include/numpy/npy_3kcompat.h b/numpy/core/include/numpy/npy_3kcompat.h
index 2d0ccd3b9..808518266 100644
--- a/numpy/core/include/numpy/npy_3kcompat.h
+++ b/numpy/core/include/numpy/npy_3kcompat.h
@@ -378,6 +378,38 @@ npy_PyFile_CloseFile(PyObject *file)
     return 0;
 }
 
+
+/* This is a copy of _PyErr_ChainExceptions, with:
+ *  - a minimal implementation for python 2
+ *  - __cause__ used instead of __context__
+ */
+static NPY_INLINE void
+npy_PyErr_ChainExceptionsCause(PyObject *exc, PyObject *val, PyObject *tb)
+{
+    if (exc == NULL)
+        return;
+
+    if (PyErr_Occurred()) {
+        /* only py3 supports this anyway */
+        #ifdef NPY_PY3K
+            PyObject *exc2, *val2, *tb2;
+            PyErr_Fetch(&exc2, &val2, &tb2);
+            PyErr_NormalizeException(&exc, &val, &tb);
+            if (tb != NULL) {
+                PyException_SetTraceback(val, tb);
+                Py_DECREF(tb);
+            }
+            Py_DECREF(exc);
+            PyErr_NormalizeException(&exc2, &val2, &tb2);
+            PyException_SetCause(val2, val);
+            PyErr_Restore(exc2, val2, tb2);
+        #endif
+    }
+    else {
+        PyErr_Restore(exc, val, tb);
+    }
+}
+
 /*
  * PyObject_Cmp
  */
diff --git a/numpy/core/src/multiarray/arrayobject.c b/numpy/core/src/multiarray/arrayobject.c
index 368f5ded7..341682588 100644
--- a/numpy/core/src/multiarray/arrayobject.c
+++ b/numpy/core/src/multiarray/arrayobject.c
@@ -1218,37 +1218,6 @@ _void_compare(PyArrayObject *self, PyArrayObject *other, int cmp_op)
     }
 }
 
-/* This is a copy of _PyErr_ChainExceptions, with:
- *  - a minimal implementation for python 2
- *  - __cause__ used instead of __context__
- */
-NPY_NO_EXPORT void
-PyArray_ChainExceptionsCause(PyObject *exc, PyObject *val, PyObject *tb)
-{
-    if (exc == NULL)
-        return;
-
-    if (PyErr_Occurred()) {
-        /* only py3 supports this anyway */
-        #ifdef NPY_PY3K
-            PyObject *exc2, *val2, *tb2;
-            PyErr_Fetch(&exc2, &val2, &tb2);
-            PyErr_NormalizeException(&exc, &val, &tb);
-            if (tb != NULL) {
-                PyException_SetTraceback(val, tb);
-                Py_DECREF(tb);
-            }
-            Py_DECREF(exc);
-            PyErr_NormalizeException(&exc2, &val2, &tb2);
-            PyException_SetCause(val2, val);
-            PyErr_Restore(exc2, val2, tb2);
-        #endif
-    }
-    else {
-        PyErr_Restore(exc, val, tb);
-    }
-}
-
 /*
  * Silence the current error and emit a deprecation warning instead.
  *
@@ -1260,7 +1229,7 @@ DEPRECATE_silence_error(const char *msg) {
     PyObject *exc, *val, *tb;
     PyErr_Fetch(&exc, &val, &tb);
     if (DEPRECATE(msg) < 0) {
-        PyArray_ChainExceptionsCause(exc, val, tb);
+        npy_PyErr_ChainExceptionsCause(exc, val, tb);
         return -1;
     }
     Py_XDECREF(exc);
@@ -1377,7 +1346,7 @@ fail:
     /*
      * Reraise the original exception, possibly chaining with a new one.
      */
-    PyArray_ChainExceptionsCause(exc, val, tb);
+    npy_PyErr_ChainExceptionsCause(exc, val, tb);
     return NULL;
 }
 
diff --git a/numpy/core/src/multiarray/buffer.c b/numpy/core/src/multiarray/buffer.c
index c8e3da8bc..51115ab53 100644
--- a/numpy/core/src/multiarray/buffer.c
+++ b/numpy/core/src/multiarray/buffer.c
@@ -989,8 +989,11 @@ _descriptor_from_pep3118_format(char *s)
     Py_DECREF(str);
     Py_DECREF(_numpy_internal);
     if (descr == NULL) {
+        PyObject *exc, *val, *tb;
+        PyErr_Fetch(&exc, &val, &tb);
         PyErr_Format(PyExc_ValueError,
                      "'%s' is not a valid PEP 3118 buffer format string", buf);
+        npy_PyErr_ChainExceptionsCause(exc, val, tb);
         free(buf);
         return NULL;
     }
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index 277f64a5d..98482bb16 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -6662,6 +6662,18 @@ class TestNewBufferProtocol(object):
             ValueError, "format string",
             np.array, m)
 
+    def test_error_message(self):
+        # wchar has no corresponding numpy type - if this changes in future, we
+        # need a better way to construct an invalid memoryview format.
+        t = ctypes.c_wchar * 4
+        with assert_raises(ValueError) as cm:
+            np.array(t())
+
+        exc = cm.exception
+        if sys.version_info.major > 2:
+            with assert_raises_regex(ValueError, "Unknown .* specifier 'u'"):
+                raise exc.__cause__
+
     def test_ctypes_integer_via_memoryview(self):
         # gh-11150, due to bpo-10746
         for c_integer in {ctypes.c_int, ctypes.c_long, ctypes.c_longlong}: