improve error handling when sql calling with (empty|unknown) host.

see #191

src/ejabberd_auth.erl

@@ -300,6 +300,9 @@ get_password_with_authmodule(User, Server) ->
 
 -spec is_user_exists(binary(), binary()) -> boolean().
 
+is_user_exists(User, <<"">>) ->
+    false;
+
 is_user_exists(User, Server) ->
 %% Check if the user exists in all authentications module except the module
 %% passed as parameter

src/ejabberd_auth_odbc.erl

@@ -212,7 +212,6 @@ is_user_exists(User, Server) ->
 		true; %% Account exists
 	    {selected, [<<"password">>], []} ->
 		false; %% Account does not exist
-	    {error, unknownhost} -> false;
 	    {error, Error} -> {error, Error}
 	  catch
 	    _:B -> {error, B}

src/ejabberd_odbc.erl

@@ -141,7 +141,7 @@ sql_call(Host, Msg) ->
     case get(?STATE_KEY) of
       undefined ->
         case ejabberd_odbc_sup:get_random_pid(Host) of
-          none -> {error, unknownhost};
+          none -> {error, <<"Unknown Host">>};
           Pid ->
             (?GEN_FSM):sync_send_event(Pid,{sql_cmd, Msg, now()},
                                        ?TRANSACTION_TIMEOUT)