bpo-36829: Enhance PyErr_WriteUnraisable() (GH-13487)

bpo-36829: Enhance PyErr_WriteUnraisable() (GH-13487)

PyErr_WriteUnraisable() now displays the exception even if displaying
the traceback failed. Moreover, hold a strong reference to sys.stderr
while using it.

Document that an exception must be set when calling
PyErr_WriteUnraisable(), but don't add an assertion to check it at
runtime.

Cleanup: use longer names for variables and create
write_unraisable_exc_file() subfunction.