Missing local messages in modules lead to InvocationTargetExceptions

[rsteph-de]

Running JHove in a Non-English environment I ran into lots of InvocationTargetExceptions if localized messages were missing.
Looking into the ErrorMessage_de.properties of different modules I discovered several issues with wrong / missing message properties.
This led to NULL values in the corresponding MessageConstants enum.

1. As a quick fix you can already set a system property to force JHove to use the English messages.

System.setProperty("module.language", "en");

2. I am going to provide some corrections to the properties as Pull Request.

3. It would be good to have a fallback to the default message, if a message is missing for a certain language.
   This will usually happen when error messages are added or updated.