patch 8.2.1281: the "trailing characters" error can be hard to understand

Problem:    The "trailing characters" error can be hard to understand.
Solution:   Add the trailing characters to the message.
diff --git a/src/sign.c b/src/sign.c
index a3dfaf0..fb452b3 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -1577,7 +1577,7 @@
 	    filename = arg;
 	    *buf = buflist_findnr((int)getdigits(&arg));
 	    if (*skipwhite(arg) != NUL)
-		emsg(_(e_trailing));
+		semsg(_(e_trailing_arg), arg);
 	    break;
 	}
 	else