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/eval.c b/src/eval.c
index bb75528..6d3d19e 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -797,7 +797,7 @@
 	if (unlet && !VIM_ISWHITE(*p) && !ends_excmd(*p)
 						    && *p != '[' && *p != '.')
 	{
-	    emsg(_(e_trailing));
+	    semsg(_(e_trailing_arg), p);
 	    return NULL;
 	}