Endpoint should check that payloads can be converted to param

[nsoblath]

File: endpoint.py
Class: Endpoint
Functions:

• do_set_request()
• do_get_request()
• do_cmd_request()

Each of those has a line that's something like:

return a_request_message.reply(payload=scarab.to_param(result))

There's no check for whether the result object can in fact be converted to param. Instead, this should be done in a try block to catch the case in which the result cannot be converted to a python object. In that case the return message should have an error return code.