more work on Scaffold base class

[davidism]

Moves all of _PackageBoundObject into Scaffold. Some related functions (that were not part of the public API) were also moved from helpers to scaffold.

errorhandler and register_error_handler both used _register_error_handler, which was redundant and didn't match the rest of the decorator/method pairs. errorhandler is now a direct wrapper around register_error_handler.

During the initial refactor to Scaffold, we missed that the various callback data was added with record_once, but the new implementation merged the data on each registration. Now it's only merged on first registration.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.