kill volatile struct gdb_exception

kill volatile struct gdb_exception

After the previous patch, this is the last remaining use of a volatile
struct gdb_exception.  Kill it, as it's troublesome for C++: we can't
assign volatile <-> non-volatile without copy constructors /
assignment operators that do that, which I'd rather avoid.

gdb/ChangeLog:
2015-03-07  Pedro Alves  <palves@redhat.com>

	* main.c (handle_command_errors): Remove volatile qualifier from
	parameter.