Return std::string from memory_error_message

Return std::string from memory_error_message

This changes memory_error_message to return a std::string and fixes up
the callers.  This removes some cleanups.

ChangeLog
2017-09-03  Tom Tromey  <tom@tromey.com>

	* valprint.c (val_print_string): Update.
	* gdbcore.h (memory_error_message): Return std::string.
	* corefile.c (memory_error_message): Return std::string.
	(memory_error): Update.
	* breakpoint.c (insert_bp_location): Update.