refactor: improve kernel error handling logic (1 of 3)

[agoose77]

For a while I've been intending upon refactoring the error handling logic for kernels, so that it's not associated with Content.

This PR does two things:

• Moves _selfless_handle_error from Content to Backend.maybe_kernel_error
• Simplifies _handle_error and renames to Content._maybe_index_error (name TBD)

[codecov]

Codecov Report

Merging #2505 (35fb169) into main (7342b79) will increase coverage by 0.08%.
The diff coverage is 92.03%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/contents/indexedarray.py	78.65% <73.33%> (ø)
src/awkward/contents/listoffsetarray.py	80.90% <76.47%> (ø)
src/awkward/_backends/backend.py	88.37% <87.50%> (-0.92%)	⬇️
src/awkward/contents/recordarray.py	85.09% <87.50%> (+0.02%)	⬆️
src/awkward/contents/indexedoptionarray.py	88.59% <93.10%> (ø)
src/awkward/contents/bytemaskedarray.py	89.07% <93.33%> (ø)
src/awkward/contents/numpyarray.py	91.09% <95.45%> (ø)
src/awkward/_reducers.py	94.20% <97.50%> (+5.43%)	⬆️
src/awkward/contents/bitmaskedarray.py	68.93% <100.00%> (ø)
src/awkward/contents/content.py	76.03% <100.00%> (+1.32%)	⬆️
... and 4 more

[jpivarski]

This is a much-needed refactoring. The kernel-error handling was getting pretty crusty.

Although it touches a lot of files, most of them are just swapping names (and making the error-handling call "selfless"). It looks straightforward enough to me that I can just merge it.