Extended Tuning page doesn't display error when parameter fails to write

The Config >> Extended tuning page doesn't seem to display an error when one of the parameters on it has been marked ReadOnly in the AP firmware. I.e. ArduPilot will not allow the parameter value to be changed but the failure to write the parameter value is not reported to the user.

To recreate the issue I did this:

- Used APJTools to mark ATC_RAT_RLL_P to be readonly
- loaded the modified firmware onto a Hex Cube Black
- Modified the parameter using the Extended Tuning page and pressed the "Write Params" button
- It appeared the parameter value was modified (the field turned from green to black and the value shown was the new value) but AP had rejected the change. Pushing the "Refresh Screen" caused the parameter value to go back to it's original value

Start by reproducing the issue in the Config >> Extended tuning page with ATC_RAT_RLL_P marked ReadOnly in the ArduPilot firmware. Use Write Params and then Refresh Screen to compare the displayed value with the firmware value; done means a rejected write is reported to the user and the field does not falsely appear committed.

Written by the indexing model from the issue text.