Better sanity checking of task IDs in start task API

[mountaindude]

Is your feature request related to a problem? Please describe.
Improved existing feature.

If an invalid task ID (such as abc) is passed to the /v4/reloadtask/:taskId/start API, Butler will log an error:

Before trying to act on a task ID it should be sanity checked, for example that the format is actually a valid guid.

Test in two places:

1. On Butler startup. Warn if an invalid task ID is found
2. Whenever a task should be started using task IDs. Warn if an invalid task ID is found, then just drop it (don't try to use it)