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_timers.vim b/src/testdir/test_timers.vim
index 338be10..85e76eb 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -265,7 +265,7 @@
   sleep 50m
   call assert_equal(3, g:call_count)
 
-  call assert_fails('call timer_start(100, "MyHandler", "abc")', 'E475:')
+  call assert_fails('call timer_start(100, "MyHandler", "abc")', 'E1206:')
   call assert_fails('call timer_start(100, [])', 'E921:')
   call assert_fails('call timer_stop("abc")', 'E39:')
 endfunc