Improve logging for preproessors

For any reason, a preprocessor is not working for me atm.
In Debug-Level, it even shows no information about preprocessors, if a non existing name is taken.

The logging should be as clear as for the reporters.

Reproduce the preprocessor problem with an invalid name at debug level and compare the output with the existing reporter logging. Trace the relevant logging entry point, then verify that preprocessor failures and missing names produce clear diagnostic information.

Written by the indexing model from the issue text.