Better messages on common exceptions.

Not part of the build.


git-svn-id: svn://10.0.0.236/trunk@115560 18797224-902f-48f8-a5cc-f745e15eee43

mozilla/extensions/python/xpcom/components.py

@@ -122,7 +122,12 @@ class _Interface:
 
 class _Interfaces(_ComponentCollection):
     def _get_one(self, name):
-        item = interfaceInfoManager.GetInfoForName(name)
+        try:
+            item = interfaceInfoManager.GetInfoForName(name)
+        except xpcom.COMException, why:
+            # Present a better exception message, and give a more useful error code.
+            import nsError
+            raise xpcom.COMException(nsError.NS_ERROR_NO_INTERFACE, "The interface '%s' does not exist" % (name,))
         return _Interface(item.GetName(), item.GetIID())
 
     def _build_dict(self):