fix errors

Author: Tobias Hafner

polypheny/rpc.py

@@ -122,7 +122,8 @@ def recv_msg(self):
             # TODO: Add to error_response something to decide if this is necessary
             # self.con.close()
             # self.con = None
-            raise Error(r.error_response.message)
+            msg = str(r.error_response.message or "An unknown error occurred.")
+            raise Error(msg)
         return r
 
     def call(self, msg):