Consolidate some Python exception-printing functions

Consolidate some Python exception-printing functions

A few places in the Python code would either call gdbpy_print_stack,
or throw a gdb "quit", depending on the pending exception.  This patch
consolidates these into a helper function.

gdb/ChangeLog
2018-12-27  Tom Tromey  <tom@tromey.com>

	* python/python-internal.h (gdbpy_print_stack_or_quit): Declare.
	* python/py-unwind.c (pyuw_sniffer): Use
	gdbpy_print_stack_or_quit.
	* python/py-framefilter.c (throw_quit_or_print_exception):
	Remove.
	(gdbpy_apply_frame_filter): Use gdbpy_print_stack_or_quit.
	* python/python.c (gdbpy_print_stack_or_quit): New function.