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_let.vim b/src/testdir/test_let.vim
index c05a4cb..d001d05 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -293,7 +293,7 @@
   let s = "var"
   let var = 1
   call assert_fails('let var += [1,2]', 'E734:')
-  call assert_fails('let {s}.1 = 2', 'E18:')
+  call assert_fails('let {s}.1 = 2', 'E1203:')
   call assert_fails('let a[1] = 5', 'E121:')
   let l = [[1,2]]
   call assert_fails('let l[:][0] = [5]', 'E708:')