fix: better exception and error handling for capsules

[Skylion007]

Description

• This solves several issues when dealing with capsules. Firstly, we were leaving the pybind11 in a bad exception state when we throw pybind11_fail without clearing the error first. We really should be throwing error_already_set to ensure that exception messages are as consistent and informative as vanilla cpython.
• There really isn't a good reason to be catching these errors so the changes should be pretty backwards compatible. In fact, a lot of the invalid error states occurred because our test suite never hit them in production.
• This guards against some nasty segfaults that could occur when mishandling capsules.
• Many of the python capsule calls only return null on error states which makes exception handling trivial. There was only one case where I had to add an additional pybind11_fail as a catch.
• Finally while browsing the repo, I found one more missing std::move and added that in.

Suggested changelog entry:

* The bindings for capsules now have more consistent exception handling.

[rwgk]

Thanks a lot for this cleanup, the revised code makes a lot more sense!
I'll try to run this through our global testing tonight, but if you need this before I have the results, please go ahead.

[rwgk]

Suggested changelogentry:
PyCapsules standing bindings now have more consistent exception handling.

standing?

standard?

Maybe: py::capsule now has more consistent exception handling.

?

[rwgk]

The test ID for the global testing is OCL:437135289:BASE:437101826:1648173046939:234ba7aa, 21:00 batch.

[rwgk]

The global testing finished without finding any issues.

[Skylion007]

@rwgk Fixed spelling errors in the PR description.

[henryiii]

Looks fantastic to me. :)