Error is misleading when user was invited already

[unRARed]

It wasn't clear from the error on the form that I had already been previously invited to the Slack org. Appears to hit line 83 of invite.go.

Screenship:

[tantalic]

@unRARed Thanks for posting an issue. I don't believe the client library we are currently (https://github.com/nlopes/slack) using exposes any machine-readable error code but will need to investigate this further. If this is the case there are two potential paths forward I see:

1. Implement better error handling within nlopes/slack such that API errors can be inspected. Hopefully this would be accepted upstream so we would not need to maintain our own fork.
2. Fetch the list of users and check for membership prior to submitting to the Slack API and check for membership.

My preference at this point would be #1, assuming it would be welcomed upstream.

[tantalic]

It should also be noted that this affects the Mail Chimp signup as well.