better error if sys.stderr is None

Issue #21497: faulthandler functions now raise a better error if
sys.stderr is None: RuntimeError("sys.stderr is None") instead of
AttributeError("'NoneType' object has no attribute 'fileno'").

Set also version to 2.4.
change headers in traceback

Issue 19306: Add extra hints to faulthandler stack dumps that they are
upside down.
set string max length to 500 characters

Python issue #15463: the faulthandler module truncates strings to 500
characters, instead of 100, to be able to display long file paths.
add _raise_signal() function

Replace _sigill() and _sigbus() functions
Merge tests from Python 3.5