Better error message when EntityManager can't be produced for injection

Describe the bug Follow up on #5521 :

when there are no entities, we automatically disable the Hibernate ORM extension, this implies that code attempting to use an injected EntityManager will get an error as the EntityManager producer is not generated either.

[error]: Build step io.quarkus.arc.deployment.ArcProcessor#validate threw an exception: javax.enterprise.inject.spi.DeploymentException: javax.enterprise.inject.UnsatisfiedResolutionException: Unsatisfied dependency for type javax.persistence.EntityManager and qualifiers [@Default]

We should find a way to produce a better error message to explain this particular UnsatisfiedResolutionException.

@mkouba could you suggest how this could be done? Does it make sense?

[jmartisk]

Maybe a dumb idea - but do we want to forbid the whole application from running in this case? If not, one option would be that the processor can log a warning and include a mock implementation of EntityManager that can be injected, but whose all methods throw a reasonable exception. Not sure what you guys think about letting the application work in this case - but to me it kinda makes sense, at least with devmode, the developer’s workflow could possibly be that they first inject an EM and only then they add an entity.

[famod]

We just had to add a Dummy entity to one of our “persistence support” Maven modules which is using sequences, to make the QuarkusTest work. This is a bit awkward. Maybe it would come in handy to enable an extension via a annotation (on the test class)?