Catch and report errors parsing contributed index files

[sandeepmistry]

Resolved #4686.

cc/ @facchinm

[ArduinoBot]

✅ Build completed.

Please test this code using one of the following:

⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-4692-BUILD-525-linux32.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-4692-BUILD-525-linux64.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-4692-BUILD-525-windows.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-4692-BUILD-525-macosx.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-4692-BUILD-525-linuxarm.tar.xz

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

[per1234]

I gave it a test with https://gist.githubusercontent.com/per1234/78bd7854b839dd71a567/raw/022b2fcc1b7f721f04623758c6fd9b5b4302e685/package_per1234_test_index.json which is missing the last closing bracket. This file causes the hourly build to no longer load once it has been downloaded but is handled correctly by the build in this PR. Thanks sandeepmistry!

[facchinm]

👍 perfect!