Remove cleanup from ada-lang.c

Remove cleanup from ada-lang.c

This removes a cleanup from ada-lang.c by having
ada_exception_message_1 return a unique_xmalloc_ptr.

ChangeLog
2018-05-21  Tom Tromey  <tom@tromey.com>

	* ada-lang.c (ada_exception_message_1, ada_exception_message):
	Return unique_xmalloc_ptr.
	(print_it_exception): Update.