Log WARN when an invalid qualifier was added via extension

See forum for original query - https://developer.jboss.org/message/984488

As it stands, Weld allows you to register a qualifier, which can be later on, during injection, deemed as invalid. This means the application will blow up anyway, only during runtime and not bootstrap.

We should add qualifier validation to BeforeBeanDiscovery.addQualifier() to stick with fail-fast approach.
We should bump the level of logging to warning (currently it is just debug).