Proposed patch makes few minor changes related to handling errors in C.

* Fixed possible NULL dereference in PyErr_NormalizeException().

* Fixed error checking in atexit._run_exitfuncs().

* Fixed possible memory leaks in _Py_FatalError_PrintExc().

* PyErr_NormalizeException() no longer recursive.

* If an error is occurred during handling other error, the original exception is chained as the __contex__ attribute to the new exception instead of just be dropped.

* PyTraceBack_Print() no longer fails OverflowError if tracebacklimit is very large negative or positive value.

* ctype error message now include the name of the initial exception instead of the repr of its class.

* Py_XDECREFs is replaced with Py_DECREFs if this is safe.

* Added few asserts.

* Other minor cleanups.