Improve error messages in LayersTest

I spent an hour yesterday figuring out what a test failure meant before writing https://github.com/wildfly/wildfly/pull/14472#issuecomment-893964187

The error messages produced by that test need to be much more verbose in order to give some context; e.g. explain that the 'test-all-layers' content is being compared to 'test-standalone-reference' (if that is the case for a particular error.)