Ensure python errors are propogated for Python->C++

[Skylion007]

Description

• Ensures that any other exceptions that occur during function overload resolution propogates to the user if possible

Suggested changelog entry:

* Propagate Python exception traceback using ``raise_from`` if a pybind11 function runs out of overloads.

[rwgk]

Looks great, thanks Aaron!
How much trouble is it to update a test to exercise this change? (The dispatcher code is very complex. Would be great to have an assurance that refactoring doesn't break this feature.)

[Skylion007]

@rwgk We would need an example to trigger it easily. (Maybe a dummy caster just sets an error and then fails to load)? One of the tests could already be triggering it, but I would need to check the __cause__ value on all the exceptions to verify.

[rwgk]

@rwgk We would need an example to trigger it easily. (Maybe a dummy caster just sets an error and then fails to load)? One of the tests could already be triggering it, but I would need to check the __cause__ value on all the exceptions to verify.

I played around a few minutes, adding assert()s. It turns out no existing test reaches the new branch. (To be sure I'm not fooling myself, I also put the assert() in the older similar code a few lines down, and that fired.)

Based on @Skylion007's comment, it seems to be quite a bit of trouble adding a test. I think it's fine to not invest the time, but it would be best to leave some low-key warning for people working on the code in the future. Here is a super simple idea:

#3673

What do you think?