Improve error message when dependent task isn't on whitelisted worker

[mitchhentges]

I recently ran a pipeline where I used a level 1 build worker that was followed by a level 3 signing worker, which failed the build due to the following error:

scriptworker.exceptions.CoTError: '"signing:parent cCstBL86QWm2XOYZyEoQxg: docker-worker ed25519 cot signature doesn\'t verify against J+PAKmq3jkS2uCpBk5WU2ycrnTFPwZujJT4OHAxm38I=: "'

Though this error describes what happened, I didn't understand it, and had to ping my team to figure out what I should do to diagnose the error.
It would be great if the error message gave a more human-understandable hint of what went wrong.

Potential solution:

• Describe signing:parent by its task name in addition to the ID, so it's more recognizable by humans
• Rather than showing the J+PAK... value, describe what it is (a whitelist of sorts?)

[escapewindow]

I recently ran a pipeline where I used a level 1 build worker that was followed by a level 3 signing worker

This should never work.

I've been trying to make the logs less verbose and more clear. Sounds like in this case, more verbosity may help make things more clear.