Catch Net::SMTPServerBusy exceptions

[atruskie]

https://github.com/QutBioacoustics/baw-workers/blob/master/lib/baw-workers/mail/mailer.rb

When many error emails are sent at once, the mailer can throw a Net::SMTPServerBusy.

This exception should be handled because:

• Exception handling code (which triggers the mailer) should not throw exceptions itself
• The SMTP exception is raised and logged (for example in the resque logs) rather than original exception which hides the problems
• and ideally, we don't want emails dropped