Gracefully handle SMTP user input errors (e.g. incorrect email addresses) to prevent Sidekiq retries
### What does this MR do?
This MR gracefully handles SMTP input errors (e.g. incorrect or invalid e-mail addresses) to prevent these types of exceptions from causing Sidekiq to retry the task. If these specific exceptions occur, they will be logged, and the e-mail will be dropped from the queue.
### Why was this MR needed?
If you include an author that has a misspelled e-mail address, Sidekiq will keep sending e-mail to all the recipients even if they have already received the e-mail. The only way to recover is to clear the Sidekiq queue.
Note that other exceptions can still be thrown (e.g. `IOError`, `Net::SMTPAuthenticationError`, `Net::SMTPServerBusy`, `Net::SMTPUnknownError`, and `TimeoutError`). If the worker encounters these, Sidekiq should retry the task.
### What are the relevant issue numbers?
Closes https://github.com/gitlabhq/gitlabhq/issues/9560
See merge request !1163
c1b490d6ef