Simplified paramLib access and show error messages for one poll cycle

Simplified getting and setting paramLib entries via a macro and created
a mechanism within poll() which makes sure that error messages are shown
for at least one poll cycle. Also moved MAXBUF_ to the SinqController
level.

src/sinqAxis.cpp

@@ -327,8 +327,17 @@ asynStatus sinqAxis::poll(bool *moving) {
     message entry which is currently in the paramLib.
     */
     if (poll_status != asynSuccess || waitingMessage[0] != '\0') {
+        /*
+        If doPoll cleared the error message paramLib entry, but an old message
+        is still waiting, set the old message.
+        */
+        getAxisParamChecked(this, motorMessageText, &newMessage);
+        if (newMessage[0] == '\0') {
+            setAxisParamChecked(this, motorMessageText, waitingMessage);
+        }
         setAxisParamChecked(this, motorStatusProblem, true);
     } else {
+        // No errors are waiting -> Clear everything.
         setAxisParamChecked(this, motorMessageText, "");
         setAxisParamChecked(this, motorStatusProblem, false);
     }

src/sinqController.cpp

@@ -495,7 +495,7 @@ asynStatus sinqController::checkComTimeoutWatchdog(sinqAxis *axis) {
     asynStatus status =
         checkComTimeoutWatchdog(axis->axisNo(), errorMessage, MAXBUF_);
     if (status == asynError) {
-        setAxisParamChecked(axis, motorMessageText, motorMessage);
+        setAxisParamChecked(axis, motorMessageText, errorMessage);
     }
     return status;
 }