Making the build fail if there is a warnings in the log

**Operating System**: `Windows`

**DocFX Version Used**: `2.56.5.0`

**Template used**: custom template

**Steps to Reproduce**:

1. Run a build on any CI build server or in a script (e.g. .ps1 or .cmd) where either `docfx metadata`, `docfx build` or `docfx pdf` produces at least one warning (e.g. because a a link cannot be resolved).

**Expected Behavior**:
The CI build fails or the script fails or there should be means to make it fail in such a situation.

**Actual Behavior**:
The CI build does not fail and there are no means to make it fail in such a situation.