Merge branch 'mojo-err-checking' into 'make_it_mojo'

This is a series of small commits to improve error handling in our Mojolicious-based backend.

The biggest changes are to import the Mojolicious debug template (which handles both 404 and 500 HTTP errors), although we also create a dedicated log file for Mojolicious to use internally in the same log directory as used for other kdesrc-build logs that run.

A direction for future work is to integrate kdesrc-build's own logging system into Mojolicious's (so that there's only one way to do it).

See merge request kde/kdesrc-build!16 (I've gone ahead and self-approved after looking at it a second time since I won't have a lot of time during the upcoming work week)