Make event properties raise AttributeError instead

They raised KeyError before. I'm changing this because the code uses
hasattr() to check for the presence of a key. This worked accidentally
before, because hasattr() silences all exceptions in python 2. However,
in python3, this isn't the case anymore.

I had a look around to see if anything depended on this raising a
KeyError and I couldn't find anything. Of course, I could have simply
missed it.

Signed-off-by: Adrian Tschira <nota@notafile.com>

synapse/events/__init__.py

@@ -48,13 +48,22 @@ class _EventInternalMetadata(object):
 
 def _event_dict_property(key):
     def getter(self):
+        try:
             return self._event_dict[key]
+        except KeyError:
+            raise AttributeError(key)
 
     def setter(self, v):
+        try:
             self._event_dict[key] = v
+        except KeyError:
+            raise AttributeError(key)
 
     def delete(self):
+        try:
             del self._event_dict[key]
+        except KeyError:
+            raise AttributeError(key)
 
     return property(
         getter,