Reconsider logging output, e.g., for test cases

[martin-fleck-at]

Currently, there are two places where we log errors directly to the console even though the test does not fail:

• EMFJsonConverterTest calls EMFJsonConverter#fromJson which logs the error even though that is expected in some test cases.
• ModelRepositoryTest calls ResourceManager#loadResource logs all resources it cannot load to the console but the ModelRepository#initialize simply iterates over all files, so many files will not be loaded as resources, e.g., Java files, preference files, etc.