timeout: Re-raise from where the TimeoutExpired hit the thread.

Having just thrown an exception into the thread, it is often useful to
know _what_ was the slow code that we interrupted.  Raising a bare
TimeoutExpired here obscures that information, as any `exc_info` will
end there.

Examine the thread for any exception information, and use that to
re-raise.  This exception information is not guaranteed to exist -- if
the thread didn't respond to the exception in time, or caught it, for
instance.

zerver/lib/timeout.py

@@ -78,11 +78,14 @@ def timeout(timeout: float, func: Callable[[], ResultT]) -> ResultT:
             time.sleep(0.1)
             if not thread.is_alive():
                 break
+        if thread.exc_info[1] is not None:
+            # Re-raise the exception we sent, if possible, so the
+            # stacktrace originates in the slow code
+            raise thread.exc_info[1].with_traceback(thread.exc_info[2])
         raise TimeoutExpired
 
     if thread.exc_info[1] is not None:
-        # Raise the original stack trace so our error messages are more useful.
+        # Died with some other exception; re-raise it
-        # from https://stackoverflow.com/a/4785766/90777
         raise thread.exc_info[1].with_traceback(thread.exc_info[2])
     assert thread.result is not None  # assured if above did not reraise
     return thread.result