Vague error for invite rate limit

Issue Summary

When the user management invite is being rate limited, the error returned is a bit misleading.

Steps to Reproduce

Send multiple invites one after the other via the site user management

Expected Results

An error message should be returned specifying to wait for a certain time period.

Actual Results

The following error is returned

Unable to add <user>
We couldn't send your invitation. Try inviting that user again

Workaround

Wait for a few minutes and try again.