Change gdbscm_exception_message_to_string to return a unique_xmalloc_ptr

Change gdbscm_exception_message_to_string to return a unique_xmalloc_ptr

This changes gdbscm_exception_message_to_string to return a
unique_xmalloc_ptr, allowing for the removal of some cleanups.
unique_xmalloc_ptr was chosen because at the root of the call chains
is a function from Guile that returns a malloc'd string.

gdb/ChangeLog
2018-07-17  Tom Tromey  <tom@tromey.com>

	* guile/scm-param.c (pascm_signal_setshow_error): Update.
	* guile/guile-internal.h (gdbscm_exception_message_to_string):
	Update.
	* guile/scm-cmd.c (cmdscm_function): Update.
	* guile/scm-pretty-print.c
	(ppscm_print_exception_unless_memory_error): Update.
	* guile/scm-exception.c (gdbscm_exception_message_to_string):
	Return unique_xmalloc_ptr.