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

[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