Make error handling more explicit, and warn user if they haven't

[benjie]

Description

If you pass a pool to Graphile Worker, that pool ought to have error handlers. We've done this for you in the past, but that can result in adding lots of error handlers to the pool. Now we only add the error handlers if there aren't already any, plus we tell you that you should be doing it yourself.

Performance impact

Negligible startup cost.

Security impact

Improvement: encouraging users to handle their own errors.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.