Warn if a Trait default method raises AttributeError

[mdickinson]

Closes #1747

This PR issues a UserWarning when a *_default trait default method raises an AttributeError, to help detect an otherwise easy-to-miss failure mode.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in stub files

[corranwebster]

Looks good to me.

Am trusting somewhat on the reference clean-up in the helper function being correct.

One suggestion is that the test seems to only test one of the two code paths for the default generation (although it is likely the most important one to catch).

[mdickinson]

the test seems to only test one of the two code paths for the default generation

Good point; fixed in 4819297

I've also suppressed an instance of the new warning from an existing test in 45a2e1d.