Merge branch 'handle-smtp-input-errors' into 'master'

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

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