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

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?)

Start by tracing the code that formats the scriptworker.exceptions.CoTError for a signing:parent signature or whitelist failure. Check how the parent task name and whitelist value are available at that point; done means the error identifies the task in human-readable terms and explains the value instead of presenting it without context.