Consolidate gdb.GdbError handling

Consolidate gdb.GdbError handling

I noticed two nearly identical copies of the same code for handling
gdb.GdbError.  The only differences were in some error messages.
These differences didn't seem very important, so this patch pulls the
code out into a new function.

2018-09-23  Tom Tromey  <tom@tromey.com>

	* python/py-function.c (fnpy_call): Use gdbpy_handle_exception.
	* python/py-cmd.c (cmdpy_function): Use gdbpy_handle_exception.
	* python/python-internal.h (gdbpy_handle_exception): Declare.
	* python/py-utils.c (gdbpy_handle_exception): New function.