Add a reason why the insert command for a new message failed

If a insert of a new message failed an error message will be written to stderr:

```
ERROR: Runtime error while trying to write
                message with message-id '%s'. That message has not been written
                to the database, but the execution has not been stopped. Please
                report this failure including the message-id and the URL for
                the mbox.
```

At first time i saw such a message i asked my "why does this message occur?".
I think it would be useful to add a reason to this message.

This PR adds the reason of the failure to the error message and writes it to stderr as well.