Make -P (pdb) work better with exceptions triggered from events

Previously, if an exception was raised from an event listener, and the
`-P` option was specified, the debugger would be started not for the
original error but for the `ExtensionError` wrapping it that was
raised by `EventManager.emit`.  That made it difficult to debug the
error.

With this change, when `-P` is specified, wrapping of errors in
`ExtensionError` is disabled, which allows pdb to debug the original
error.

tests/test_events.py

@@ -19,11 +19,16 @@ def test_event_priority():
     assert result == [3, 1, 2, 5, 4]
 
 
+class FakeApp:
+    def __init__(self, pdb: bool = False):
+        self.pdb = pdb
+
+
 def test_event_allowed_exceptions():
     def raise_error(app):
         raise RuntimeError
 
-    events = EventManager(object())  # pass an dummy object as an app
+    events = EventManager(FakeApp())  # pass an dummy object as an app
     events.connect('builder-inited', raise_error, priority=500)
 
     # all errors are converted to ExtensionError
@@ -33,3 +38,19 @@ def test_event_allowed_exceptions():
     # Allow RuntimeError (pass-through)
     with pytest.raises(RuntimeError):
         events.emit('builder-inited', allowed_exceptions=(RuntimeError,))
+
+
+def test_event_pdb():
+    def raise_error(app):
+        raise RuntimeError
+
+    events = EventManager(FakeApp(pdb=True))  # pass an dummy object as an app
+    events.connect('builder-inited', raise_error, priority=500)
+
+    # errors aren't converted
+    with pytest.raises(RuntimeError):
+        events.emit('builder-inited')
+
+    # Allow RuntimeError (pass-through)
+    with pytest.raises(RuntimeError):
+        events.emit('builder-inited', allowed_exceptions=(RuntimeError,))