Don't Try to Send E-mail when over quota

[dhermes]

From the quota docs:

try:
  mail.SendMessage(to='test@example.com',
                   from='admin@example.com',
                   subject='Test Email',
                   body='Testing')
except apiproxy_errors.OverQuotaError, message:
  # Log the error.
  logging.error(message)
  # Display an informative message to the user.
  self.response.out.write('The email could not be sent. '
                          'Please try again later.')