build_meta: produce informative error when a dist is not found

Previously, when `build_sdist` or `build_wheel` were unable
to build a distribution (and were therefore unable to find the
distribution file), they would throw a

    ValueError: not enough values to unpack (expected 1, got 0)

which did not offer any clues as to where the issue might lie.

1 files changed, 6 insertions, 2 deletions

diff --git a/setuptools/build_meta.py b/setuptools/build_meta.py
index b9e8a2b3..3c45db72 100644
--- a/setuptools/build_meta.py
+++ b/setuptools/build_meta.py
@@ -101,8 +101,12 @@ def _file_with_extension(directory, extension):
         f for f in os.listdir(directory)
         if f.endswith(extension)
     )
-    file, = matching
-    return file
+    try:
+        return next(matching)
+    except StopIteration:
+        raise ValueError('No distribution was found. The distribution was '
+                         'possibly not built. Ensure that your `setup.py` '
+                         'is not empty and that it calls `setup()`.')
 
 
 def _open_setup_script(setup_script):