Create new exception for boot errors

This expands on discussions from WFCORE-6090 and the PR for WFCORE-6096. The main goal is to create a subclass of OperationFailedException for boot errors, and throw it in place of a fallback exception:

As suggested by bstansbe@redhat.com:

I've thought about it a bit and this would be really helpful:

1. Make BootErrorCollector.getErrors() package protected. (BTW, I think the entire class can and should be package protected, but if there's some outside use I've missed, then never mind.)
2. Add to AbstractControllerService a 'protected final ModelNode getBootErrors()' that just exposes the BEC.getErrors() result.
3. Create a new private static subclass of OperationFailedException in ModelTestControllerService. Instead of throwing your unknownBootFailure constant, create and throw an instance of it using the result from the new getBootErrors(). In waitForSetup use an instance of check.