Refactor bridgedb.email.server.MailMessage.getRecipient().

 * CHANGE to using only one try/except block for
   `smtp.AddressError`s. We should probably assume that our
   EMAIL_SMTP_FROM_ADDR in the config file is correctly formed, and not
   bother logging the special case if it is not. Instead, just log all
   `smtp.AddressError`s in the same manner.
 * CHANGE variable names to make code clearer.