Make core:warnings_as_errors accept a list

[AbrilRBS]

Changelog: Fix: Make core:warnings_as_errors accept a list
Docs: Omit

WIP, uploading to get it out as soon as it's done :)

[memsharded]

Looks good, please check:

def error(self, msg, error_type=None):
        if self._conan_output_level <= LEVEL_ERROR:
            if self._warnings_as_errors and error_type != "exception":
                raise ConanException(msg)

I wouldn't condition the exception to the log-level LEVEL_ERROR. Even if it is not logged, we want it to raise and report it.