patch 8.2.2481: Vim9: confusing error when variable arguments have default

Problem:    Vim9: confusing error when variable arguments have a default
            value.
Solution:   Give a specific error message. (closes #7793)

src/testdir/test_vim9_func.vim

@@ -781,6 +781,16 @@ def Test_call_def_varargs()
       Func(1, 'a')
   END
   CheckScriptFailure(lines, 'E1013: Argument 1: type mismatch')
+
+  lines =<< trim END
+      vim9script
+      def Func(  # some comment
+                ...l = []
+                )
+        echo l
+      enddef
+  END
+  CheckScriptFailure(lines, 'E1160:')
 enddef
 
 let s:value = ''

src/userfunc.c

@@ -241,6 +241,11 @@ get_function_args(
 									 skip);
 		if (p == arg)
 		    break;
+		if (*skipwhite(p) == '=')
+		{
+		    emsg(_(e_cannot_use_default_for_variable_arguments));
+		    break;
+		}
 	    }
 	}
 	else

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2481,
 /**/
     2480,
 /**/