[mythtv] webapp settings error checking

[Peter Bennett]

I have added error checking to the miscellaneous settings tab. The 
observable supports three callbacks. The error callback is called if 
there is an http error.

I added a div to the html page for the confirmation or error message.

My screen layout and usage of classes may not be best. The footer that I 
copied used classes p-grid p-nogutter p-justify-end. These caused very 
strange things to happen when I tried to add text between the buttons. I 
changed them to grid and nogutter and added col-1 instead of 
p-justify-end. These are according to the documentation in 
https://primefaces.org/primeflex/gridsystem .

I don't know where the p-grid etc are documented.

The text color classes in https://primefaces.org/primeflex/textcolor did 
not work, I don't know why, so I used style="color:red;"

Please feel free to let me know if I have gone wrong and where. If you 
prefer the p-grid etc, where are they documented?

Peter