Refactor exceptions.

We currently have only one Exception in the facetapi namespace: \Drupal\facetapi\Exception. To add more verbose Exceptions we should add new exceptions for specific use cases.

One such usecase (that would be a great start) is introducing a new \Drupal\facetapi\Exception\InvalidProcessorException that extends the current \Drupal\facetapi\Exception.

This new exception should be thrown where currently the facet api exception is thrown in the facet manager.