builtin-apply: Show a more descriptive error on failure when opening a patch

When a patch can't be opened (it doesn't exist, there are permission
problems, etc.) we get the usage text, which is not a proper indication of
failure.

Signed-off-by: Alberto Bertogli <albertito@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-apply.c

@@ -2888,7 +2888,7 @@ int cmd_apply(int argc, const char **argv, const char *unused_prefix)
 
 		fd = open(arg, O_RDONLY);
 		if (fd < 0)
-			usage(apply_usage);
+			die("can't open patch '%s': %s", arg, strerror(errno));
 		read_stdin = 0;
 		set_default_whitespace_mode(whitespace_option);
 		errs |= apply_patch(fd, arg, inaccurate_eof);