Warning or Error for unknown keywords in configuration file

[GPMueller]

It would be helpful to get a warning or error that an unknown keyword was found in the configuration file.

Since the feature-set and keywords of clang-build are not trivial anymore, one might expect to be able to use a keyword in a place that we have not designed it for, or the ordering of some nested structure might be wrong.
It might also be that a user misspelled a keyword or upgraded clang-build to a version that changed keywords.