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_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index 0c1e75d..1fbb74b 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
@@ -165,7 +165,7 @@
 
   call assert_fails('echo a . b', 'E15:')
   call assert_fails('let a .= b', 'E985:')
-  call assert_fails('let vers = 1.2.3', 'E15:')
+  call assert_fails('let vers = 1.2.3', 'E488:')
 
   if has('float')
     let f = .5