Simplify exception handling in py-framefilter.c

This patch changes py-framefilter.c as suggested by Pedro in:
https://sourceware.org/ml/gdb-patches/2017-06/msg00748.html

In particular, gdb exceptions are now caught at the outermost layer,
rather than in each particular function.  This simplifies much of the
code.

gdb/ChangeLog
2018-03-26  Tom Tromey  <tom@tromey.com>

	* python/py-framefilter.c (py_print_type): Don't catch
	exceptions.  Return void.
	(py_print_value): Likewise.
	(py_print_single_arg): Likewise.
	(enumerate_args): Don't catch exceptions.
	(py_print_args): Likewise.
	(py_print_frame): Likewise.
	(gdbpy_apply_frame_filter): Catch exceptions here.