Mojo err checking

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).