Close #8813: Show what extension caused it on errors on event handler

Show the module name of the event handler on the error inside it to let
users know a hint of the error.

3 files changed, 13 insertions, 3 deletions

diff --git a/CHANGES b/CHANGES
index a4ab8bd51..9e3e29fe0 100644
--- a/CHANGES
+++ b/CHANGES
@@ -67,6 +67,7 @@ Features added
   dedent via no-argument ``:dedent:`` option
 * C++, also hyperlink operator overloads in expressions and alias declarations.
 * #8247: Allow production lists to refer to tokens from other production groups
+* #8813: Show what extension (or module) caused it on errors on event handler
 
 Bugs fixed
 ----------
diff --git a/sphinx/errors.py b/sphinx/errors.py
index c632d8dbc..3e84b6b88 100644
--- a/sphinx/errors.py
+++ b/sphinx/errors.py
@@ -47,12 +47,19 @@ class ApplicationError(SphinxError):
 
 class ExtensionError(SphinxError):
     """Extension error."""
-    category = 'Extension error'
 
-    def __init__(self, message: str, orig_exc: Exception = None) -> None:
+    def __init__(self, message: str, orig_exc: Exception = None, modname: str = None) -> None:
         super().__init__(message)
         self.message = message
         self.orig_exc = orig_exc
+        self.modname = modname
+
+    @property
+    def category(self) -> str:  # type: ignore
+        if self.modname:
+            return 'Extension error (%s)' % self.modname
+        else:
+            return 'Extension error'
 
     def __repr__(self) -> str:
         if self.orig_exc:
diff --git a/sphinx/events.py b/sphinx/events.py
index 806a6e55d..e3a3b964f 100644
--- a/sphinx/events.py
+++ b/sphinx/events.py
@@ -19,6 +19,7 @@ from sphinx.deprecation import RemovedInSphinx40Warning
 from sphinx.errors import ExtensionError, SphinxError
 from sphinx.locale import __
 from sphinx.util import logging
+from sphinx.util.inspect import safe_getattr
 
 if False:
     # For type annotation
@@ -114,8 +115,9 @@ class EventManager:
             except SphinxError:
                 raise
             except Exception as exc:
+                modname = safe_getattr(listener.handler, '__module__', None)
                 raise ExtensionError(__("Handler %r for event %r threw an exception") %
-                                     (listener.handler, name), exc) from exc
+                                     (listener.handler, name), exc, modname=modname) from exc
         return results
 
     def emit_firstresult(self, name: str, *args: Any,