patch 8.2.3055: strange error for assigning to "x.key" on non-dictionary

Problem:    Strange error for assigning to "x.key" on non-dictionary.
Solution:   Add a specific error message. (closes #8451)
diff --git a/src/testdir/test_listdict.vim b/src/testdir/test_listdict.vim
index 8351acb..96debd6 100644
--- a/src/testdir/test_listdict.vim
+++ b/src/testdir/test_listdict.vim
@@ -294,6 +294,9 @@
   let d.1 = 1
   let d._ = 2
   call assert_equal({'1': 1, '_': 2}, d)
+
+  let n = 0
+  call assert_fails('let n.key = 3', 'E1203: Dot can only be used on a dictionary: n.key = 3')
 endfunc
 
 " Function in script-local List or Dict