Various fixes to stability

[c-schmitz]

We are processing a huge number of bounces daily, and we have been starting to use this library recently.
We found some notices happening and made according fixes.

One issue was that the status code was plain empty in some message, even though the field existed.
In that case the necessary information could still be pulled from the Diagnostic code field

Another issue was that the Diagnostic code would not contain a text separated with semikolon, but a full status text.

If you have any questions, please let me know.

[marclaporte]

@c-schmitz Are you using this in production? If so, given your use case, it's surely good to merge.

Thanks!