patch 8.2.3016: confusing error when expression is followed by comma

Problem:    Confusing error when expression is followed by comma.
Solution:   Give a different error for trailing text. (closes #8395)
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 9237fdf..4b0ab2b 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -3212,7 +3212,7 @@
       'if 1# comment3',
       '  echo "yes"',
       'endif',
-      ], 'E15:')
+      ], 'E488:')
 
   CheckScriptFailure([
       'vim9script',
@@ -3221,7 +3221,7 @@
       'elseif 2#comment',
       '  echo "no"',
       'endif',
-      ], 'E15:')
+      ], 'E488:')
 
   CheckScriptSuccess([
       'vim9script',
@@ -3231,7 +3231,7 @@
   CheckScriptFailure([
       'vim9script',
       'var v = 1# comment6',
-      ], 'E15:')
+      ], 'E488:')
 
   CheckScriptSuccess([
       'vim9script',