Implement smtp logging into /var/log/sendgrid.log and retry on failure

[leigh-ols]

As per my comment in issue #71 Note that /var/log/sendgrid.log will need to be created and made http server writable by the user.

The solution isn't perfect, as 3 consecutive failures will still result in emails disappearing into the ether. However it should minimize this and at least provide logs of failures for users. And so work as a useful stop gap until a proper mail queuing system can be implemented.