Error should be raised by client or post when a request for a function that doesn't exist or missed kwargs

Currently missing functions or incorrect arguments raise an error at the server side, and then throw errors on slack. These errors should only occur if the actual function has failed

Start by tracing how the client or post path validates function names and kwargs before requests reach the server, then reproduce both failure cases. Done means missing functions and incorrect arguments raise on the client or post path, while Slack errors occur only when the actual function fails.

Written by the indexing model from the issue text.