Fix crash due to index json parse errors and permission problems

[cmaglie]

See #7937

[PaulStoffregen]

Good: The IDE is able to start up after I change the first character in library_index.json from '{' to '<'.

Good: Opening the Library Manager causes library_index.json to be updated.

Not so good: After updating library_index.json, the library manager window remains blank. It does display properly if I close and reopen the library manager window.

I tested with arduino-PR-7964-BUILD-769 on Linux 64 bit.

[PaulStoffregen]

Here's the library manager window after it has replaced library_index.json with a good copy.

The library list actually has been updated, but a user looking only at the GUI and not the file in the hidden folder would believe it didn't do anything.

[cmaglie]

Yes, it has to re-open the window, I'm trying to fix this.

[cmaglie]

Pushed other two commits, should fix it for good

[ArduinoBot]

✅ Build completed.

Please test this code using one of the following:

⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-7964-BUILD-771-linux32.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-7964-BUILD-771-linux64.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-7964-BUILD-771-windows.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-7964-BUILD-771-macosx.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-7964-BUILD-771-linuxarm.tar.xz

ℹ️ The linuxarm build is still experimental and may not be always available.

[PaulStoffregen]

Confirmed with PR-7964-BUILD-771, the GUI is properly updating. Looks great!