Fix error reporting for bad Luxi arguments in QueryD

Currently, the query daemon would simply close the connection to the
client without issuing a response, if parsing the arguments failed;
the error was just logged.

Since this is very ugly from the client's point of view, we change it
so that the error is both logged and sent back to the
client. Furthermore, the execution block reporting is also cleaned up
a bit, to make the code more uniform.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>

htools/Ganeti/Queryd.hs

@@ -139,9 +139,10 @@ handleClientMsg client creader args = do
   call_result <- handleCallWrapper cfg args
   (!status, !rval) <-
     case call_result of
-      Bad x -> do
-        logWarning $ "Failed to execute request: " ++ x
-        return (False, JSString $ J.toJSString x)
+      Bad err -> do
+        let errmsg = "Failed to execute request: " ++ err
+        logWarning errmsg
+        return (False, showJSON errmsg)
       Ok result -> do
         logDebug $ "Result " ++ show (pp_value result)
         return (True, result)
@@ -159,8 +160,11 @@ handleClient client creader = do
                      return False
     RecvOk payload ->
       case validateCall payload >>= decodeCall of
-        Bad err -> logWarning ("Failed to parse request: " ++ err) >>
-                   return False
+        Bad err -> do
+             let errmsg = "Failed to parse request: " ++ err
+             logWarning errmsg
+             sendMsg client $ buildResponse False (showJSON errmsg)
+             return False
         Ok args -> handleClientMsg client creader args
 
 -- | Main client loop: runs one loop of 'handleClient', and if that