Suppress null pointer exceptions from custom validators

The CustomValidatorManager class currently logs all exceptions that occur in custom validators. This is in contrast to standard validators where null pointer exceptions are suppressed by Xtext, see AbstractDeclarativeValidator#handleExceptionDuringValidation.

Unfortunately, the genmodel validator throws many different null pointer exceptions on incomplete/broken models and it would be difficult and time consuming to fix all of them. As a result, the error log is polluted with countless null pointer exceptions from the genmodel validator.

This commit removes the additional logging logic for custom validators. Null pointer exceptions in custom validators are thus suppressed via the standard Xtext exception handling.

An alternative could be to remove the genmodel validator entirely. Those errors would then only be reported when trying to generate a model.