messages: Replace plain \n with full CRLF

A VteTerminal expects newlines to be done with \r\n (CRLF), whereas
most programs output \n on Unix OSes, so if errors are passed from
called programs to the messages panel, then they'll appear strangely e.g
abcdef
      ghijkl
            mnop
instead of:
abcdef
ghijkl
mnop
1 job for rustfmt-beautifier in 29 minutes and 39 seconds