Help clients deal with errors

[idgetto]

There's a ton of different Slack errors that clients have to deal with. In most cases the particular error isn't important, but the category is. It would be really helpful to know if an error is a client error or a server error. This would let us retry or just fail requests based on the category of error.

@darcatron
@szabowexler

[szabowexler]

I think a more friendly version of the errors would include the broad class (as you're describing) but also the snippet of text that slack gives us. It'll be time intensive but really helpful to pull all that in.

So: