Describe the bug
Given something that produces a warning, like \abrief foo (no such command 'abrief'), I would expect that WARN_AS_ERROR = FAIL_ON_WARNINGS should give me output that reflected that failure (i.e. turn them into errors).

Instead, I get this:
foo.h:16: warning: Found unknown command '\abrief'

Expected behavior
WARN_AS_ERROR = yes produces this, which I think is better:

foo.h:16: error: Found unknown command '\abrief' (warning treated as error, aborting now)

Version
Doxygen 1.9.1 on Fedora 34, distribution provided (but I have a hard time believing that the platform is relevant).