workflows: improvements and fixes

• Adds a new workflow for harvesting workflow with BibSched integration.

• Updates several workflow tasks used by the above workflow.

• Makes the BibWorkflowEngine function to return the current task/function name a bit more robusts and renames it from getCurrTaskName to get_current_taskname.

• Adds a new file errors.py that contains all custom workflows exceptions as per the new standard.

• Improves the exception handling in BibWorkflowEngine and client.py.

• Several updates in the generic_harvesting_workflow.

• The workflow tasks in marcxml_tasks are now using the OaiHarvest models correctly in terms of loading arguments deserialized.

• Removes duplicate code in client.py.

• Updates demosite regression tests.

• Fixes some Babel string problems.

Co-authored-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Co-authored-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>
Co-authored-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>