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/errors.h b/src/errors.h
index 9d987c9..1784430 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3318,3 +3318,5 @@
 EXTERN char e_can_only_use_left_padding_when_column_is_zero[]
 	INIT(= N_("E1296: Can only use left padding when column is zero"));
 #endif
+EXTERN char e_non_null_dict_required_for_argument_nr[]
+	INIT(= N_("E1297: Non-NULL Dictionary required for argument %d"));