ensure generation of valid EDSP error stanzas

ensure generation of valid EDSP error stanzas

The crude way of preparing a message to be a multiline value failed at
generation valid deb822 in case the error message ended with a new line
like the resolving errors from apt do. apt itself can parse these, but
other tools like grep-dctrl choke on it, so be nice and print valid.

Reported-By: Johannes 'josch' Schauer on IRC