HRW: Produce errors when bad mods are used

[zwoop]

This produces errors in the log output, but they are not considered load / reload failures. I.e. ATS will still startup as before. We should fix this in ATS v11.

Before this, configs like this would silently succeed, with the bad modifier ignored:

cond %{CLIENT-HEADER:X-Foo} ="foo" [BAD]