Improve DefinitionProvider#getErrorMessages() API with respect to the fact that M2B now returns more sophisticated Problem objects.

Collect problems from conversion result produced by M2B (in YamlDefinitionProvider#resolve). Also provide fallback support for the old API.

At the same time we should adjust the validity criteria of a definition provider: currently DefinitionProviderBuilder.DefinitionProviderImpl#DefinitionProviderImpl() assumes any error message makes provider invalid, but we could encounter also just a warning => I suggest to iterate over the newly introduced problems and check their types searching for the the most severe ones.