Optionally propagate SMTP errors

[Hermsi1337]

This adds an option to pass SMTP call errors back to the caller instead of only logging them.
Useful for integrations and tests that need the actual error instead of just logs.

Heads-up: my IDE snuck in a few formatting changes I couldn’t fully tame. Let me know if you want me to clean those up.

[axllent]

Thanks @Hermsi1337. It's a bit unfortunate about the formatting issues, but that's not too serious and can be rectified if needed :) Two further requests please:

1. You've added this option for relaying, but not for forwarding. Those features are very closely linked (and work very similar), so if one option gets this feature then I strongly believe the other should have it too.
2. If you wouldn't mind creating a PR for the website documentation too in those two above-mentioned links? If that's not possible then I can sort that out, but it would make your new feature complete.

Thank you again for your contribution!

[Hermsi1337]

Thanks for you fast reply, I hope you enjoyed your WE.

I've added the requested changes as well as updated the docs: axllent/mailpit-website#24

[axllent]

Thank you for making those changes (including the website documentation) @Hermsi1337 - I'll see if I can get a new release out very soon (I'm just struggling with a broken upstream dependency for some Go updates which I'm also trying to resolve).