fix for lp712982, and likely a variety of other dashboard error handling

issues.  This fix simply causes the default error code for ApiError
to be 'ApiError' rather than 'Unknown', which makes dashboard handle
the error gracefully, and makes euca error output slightly prettier

nova/exception.py

@@ -46,7 +46,7 @@ class Error(Exception):
 
 
 class ApiError(Error):
-    def __init__(self, message='Unknown', code='Unknown'):
+    def __init__(self, message='Unknown', code='ApiError'):
         self.message = message
         self.code = code
         super(ApiError, self).__init__('%s: %s' % (code, message))