patch 9.0.0335: checks for Dictionary argument often give a vague error

Problem:    Checks for Dictionary argument often give a vague error message.
Solution:   Give a useful error message. (Yegappan Lakshmanan, closes #11009)
diff --git a/src/testdir/test_map_functions.vim b/src/testdir/test_map_functions.vim
index d8c079c..79321a0 100644
--- a/src/testdir/test_map_functions.vim
+++ b/src/testdir/test_map_functions.vim
@@ -273,7 +273,7 @@
   bwipe!
 
   call assert_fails('call mapset([], v:false, {})', 'E730:')
-  call assert_fails('call mapset("i", 0, "")', 'E715:')
+  call assert_fails('call mapset("i", 0, "")', 'E1206:')
   call assert_fails('call mapset("i", 0, {})', 'E460:')
 endfunc