Description

This adds a handler to execute code on completion with or without error. This
arose from a specific use-case described here, however
can be a useful feature for running post-workflow routines (e.g. cleaning cache directories).

QC

• The PR contains a test case for the changes or the changes are already covered by an existing test case.
  There are no tests for other handlers, so not sure what to add as a test.
• The documentation (docs/) is updated to reflect the changes or this is not necessary (e.g. if the change does neither modify the language nor the behavior or functionalities of Snakemake).