logfile: Improve Output Message

[markuslf]

Describe the solution you'd like
Today, the logfile check reports Scanned 0 lines. This is a bit misleading. It would be better to be a bit more verbose and to report Scanned /var/log/duplicity.log using patterns 'ERROR' (matched 0 lines) and 'WARNING' (matched 0 lines), ignoring 'ERROR 13'.

Also, instead of Scanned 0 lines, 12 old critical matches, report Scanned /var/log/duplicity.log using patterns 'ERROR' (matched 12 lines) [CRITICAL] and 'WARNING' (matched 0 lines), ignoring 'ERROR 13'.