Indexer: get rid of exception programming in the main flow

Currently in parts of the code there are exceptions, which are used as main program flow instead of a real exception case, which is bad practice, and probably slows down the indexer.

One example of this is the 'UnhandledDispatchException' (https://codebase.helmholtz.cloud/hmc/hmc-public/unhide/data_harvesting/-/blob/dev/data_harvesting/indexer/indexer.py#L653), which triggers for each data indexed because the dispatch function deals with special cases which have to be handled while the usual cases, like Person, CreativeWork, Organization, have no special treatment and run into the exception....

Solution:

Instead of an exception, introduce a default case handler and call the dispatch with this type for certain types instead. or explicitly add the same code for each type to point out how they are dispatched.

The exception should ONLY catch special cases, which should have a treatment but do not have one implemented.