Be specific about which exceptions to rescue

The only exception we are expecting is a Mail::Field::ParseError and so this
is the only exception we should rescue from.

Rescuing from other exceptions may mask other problems for example in issue #33
when all emails were being marked as invalid.
Stop rescuing exceptions in `EmailValidator#validate_each`

Because there don't seem to be any exceptions that we are expecting and would
genuinely want to rescue from.
Merge pull request #39 from mocoso/stop-rescuing-all-exceptions

Stop rescuing all exceptions