Raising an error and exit with the tombo_helper function should return non-zero value

[patbohn]

As mentioned in the title, when an error is raised via tombo_helper.error_message_and_exit() the sys.exit() call (see below) should return a non-zero value to ensure that errors are properly caught by workload managers.

https://github.com/nanoporetech/tombo/blob/afdb4d87249806d3a171d009f948f5b8558d35a0/tombo/tombo_helper.py#L361