Non-matching calls to PyErr_Fetch/Restore cause a leak in v3.12,
so ensure calls are symmetrical or avoid where possible.
Simplify extraction of the exception buffer.
- Only overwrite the stderr (the stdio isn't used).
- Simplify pyc_exception_buffer_handle_system_exit usage.
- Remove goto's.
Also simplify calling conventions for PyC_ExceptionBuffer functions.
- They must be called when an error has occurred.
- Always return a string, never null since a null return value would
only happened in rare/unexpected cases which wasn't being checked
for by some callers, leading to potential crashes.
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