Fail the docs generation (in author-mode) on Warnings

[ignasi35]

AsciiDoc supports lowering the failure of the processor using --failure-level=LEVEL (defaults to FATAL).

~It'd be useful to setup --failure-level=WARNING. I've been looking around but can't locate how to setup our Makefile/antora build to pass that setting down into the asciidoctor command.~ Edit: that setting is for asciidoctorCLI, not asciidcotor.js.

[ignasi35]

I asked on the antora Gitter channel:

the Asciidoctor logging infrastructure was introduced in Asciidoctor 2+ and Antora is using Asciidoctor 1.5+ once antora/antora#522 will be resolved, it will be technically possible to retrieve the "maximum" log level from the logger until then you can probably hack something based on the stdout of Antora using some regular expressions