Give a proper SCSCP error instead of closing the connection when it is unable to parse the OM message

[olexandr-konovalov]

As @defeo mentioned in OpenMath/py-scscp#3, the server should give a proper SCSCP error instead of closing the connection when it is unable to parse the OM message.

[olexandr-konovalov]

I do think that parsing occurs inside CALL_AND_CATCH and the error message is caught and sent to the client - would appreciate exact example from @defeo to check what happened.