Raise fatal font errors in Mux task in log output

[begna112]

When a font isn't found (fatal error), it should actually indicate that it is an error in the log output, rather than "warning". That should make it easier for the user to identify which fonts are causing issues. In a case where lots of fonts are being used, this can save significant time.

Validating fonts for Maid Dragon S2 - 04 [TV Final].ass...
warning: font 'KBPlanetEarth' not found on line(s): 8795 8799 8802 8805 8808 8811 8814 8817 8818 8821
warning: faux bold used for font AltonaBold (requested weight 400, got 5) on line(s): 4871 4872 4873 4874 4875 4888 4889 4890 
.............

> Task :mux.04.TV FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':mux.04.TV'.
> one or more fatal font-related issues encountered

should respect whatever settings are specified with the following, if not default:

onMissingGlyphs(ErrorMode.WARN)
onMissingFonts(ErrorMode.FAIL)