Fix codecs.register_error callback type

[stroxler]

The callback receives one of the three concrete Unicode codec error types, not just UnicodeError

Model that union so contextual typing exposes their shared attributes. I found this because an engineer using Pyrefly ran into a bug where Pyrefly reported attribute errors on the use of e.start / e.end / e.object as in

import codecs

codecs.register_error(
    "zsh", lambda e: ("".join(f"<{b:02x}>" for b in e.object[e.start : e.end]), e.end)
)

[AlexWaygood]

thank you!

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

scipy-stubs (https://github.com/scipy/scipy-stubs)
- tests/misc/test_submodules_empty.pyi:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

[AlexWaygood]

thank you!