Restore compat with Autorest TestServer

msrest/exceptions.py

@@ -134,13 +134,17 @@ class HttpOperationError(ClientException):
                 self.error = deserialize(resp_type, response)
                 if self.error is None:
                     self.error = deserialize.dependencies[resp_type]()
-                # ARM uses OData v4
+                # ARM uses OData v4, try that by default
                 # http://docs.oasis-open.org/odata/odata-json-format/v4.0/os/odata-json-format-v4.0-os.html#_Toc372793091
                 # Code and Message are REQUIRED
-                self.message = "({}) {}".format(
-                    self.error.error.code,
-                    self.error.error.message
-                )
+                try:
+                    self.message = "({}) {}".format(
+                        self.error.error.code,
+                        self.error.error.message
+                    )
+                except AttributeError:
+                    # Try the default for Autorest if not available (compat)
+                    self.message = self.error.message
         except (DeserializationError, AttributeError, KeyError):
             pass