Remove aws-ses and exception_notification gems

These were used for exception notification emails, but we're now
using Errbit for tracking exceptions so don't need these any more.
Don't rescue StandardError to send an email

We are now using Errbit for tracking exceptions, and have already seen
an error related to Redis memory usage show up in Errbit from the
DNS details import, so I presume that we don't need to explicitly
do anything to be notified of errors in the Whitehall mappings import
either.
Merge pull request #208 from alphagov/remove_exception_emails

Remove exception emails