ENH: Skip Node execution propagation

[cpelley]

I have previously added the SkipBranch exception which may allow some capability for skipping node execution but this precludes serial schedulers. The approach applied here is to have a plugin return a SKIP_EVENT signal which is in turn propagated throughout successive nodes.

• Added SKIP_EVENT singleton which indicates to the plugin_executor to skip execution. This then allows propagation along the execution network.

Additional minor fixes:

• In-house multiprocessing error handling message update (to reflect new call UI).
• integration test temporary file location scope corrected (now different for each test as it intended to have been).

Issues

• https://github.com/MetOffice/improver_suite/issues/2253
• https://github.com/MetOffice/improver_suite/issues/2079

[cpelley]

of likely interest to you @JoshuaWiggs