Log exceptions when failing to auto-join new user according to the `a…

…uto_join_rooms` option. (element-hq#17176)

Would have been useful for tracking down element-hq#16878.

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>

changelog.d/17176.misc

@@ -0,0 +1 @@
+Log exceptions when failing to auto-join new user according to the `auto_join_rooms` option.

synapse/handlers/register.py

@@ -590,7 +590,7 @@ async def _join_rooms(self, user_id: str) -> None:
                 # moving away from bare excepts is a good thing to do.
                 logger.error("Failed to join new user to %r: %r", r, e)
             except Exception as e:
-                logger.error("Failed to join new user to %r: %r", r, e)
+                logger.error("Failed to join new user to %r: %r", r, e, exc_info=True)
 
     async def _auto_join_rooms(self, user_id: str) -> None:
         """Automatically joins users to auto join rooms - creating the room in the first place