mod_block_strangers: Bounce an error message

Return a stanza error when a message is rejected, in order to make
legitimate users aware of the issue.

Closes processone#2197.

src/mod_block_strangers.erl

@@ -95,7 +95,7 @@ check_message(#message{from = From, to = To} = Msg) ->
 		    if
 			Log ->
 			    ?INFO_MSG("~s message from stranger ~s to ~s",
-				      [if Drop -> "Dropping";
+				      [if Drop -> "Rejecting";
 					  true -> "Allow"
 				       end,
 				       jid:encode(From), jid:encode(To)]);
@@ -104,6 +104,9 @@ check_message(#message{from = From, to = To} = Msg) ->
 		    end,
 		    if
 			Drop ->
+			    Txt = <<"Messages from strangers are rejected">>,
+			    Err = xmpp:err_policy_violation(Txt, ?MYLANG),
+			    ejabberd_router:route_error(Msg, Err),
 			    deny;
 			true ->
 			    allow