Replace "Unknown internal error occurred" with a more helpful message.

[rwgk]

Description

This PR extracts one particular pair of related behavior changes from PR #1895, to make the development history easier to follow.

• For the public error_already_set, the usual net observable behavior change is simply a changed message; see changes in test_exceptions.py. Internally the detour through an artificially generated Python exception is eliminated, a std::runtime_error is thrown immediately from the error_already_set constructor. The rationale is that the root issue is obviously a C++ coding error.

• The non-public detail::error_string() no longer has the surprising side-effect of setting an "Unknown internal error occurred" Python RuntimeError if the Python error indicator is not actually set when the function is called. A std::runtime_error is thrown instead. Again, the rationale is that the root issue is obviously a C++ coding error.

The source code comments are updated to reflect the behavior changes. While at it, existing behavior & limitations are more fully documented.

Note that these behavior changes passed Google-global testing via PR #1895, without any adjustments to user code. The main benefit of this PR is to improve the development experience (debugging).

Suggested changelog entry:

The behavior or ``error_already_set`` was made safer and the highly opaque "Unknown internal error occurred" message was replaced with a more helpful message.