arduino_ci: Build failure and cryptic error when arduino binary fails to list its preferences

System

• OS: GitLab/Linux
• ruby -v: ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux-gnu]
• bundle -v: Bundler version 1.16.1
• bundle info arduino_ci: Installing arduino_ci 0.2.0
• Arduino IDE version:
• java -version:
• URL of failing CI job: https://gitlab.com/robotics_fun/DC_motors/-/jobs/260929416
• URL of your Arduino project: https://gitlab.com/robotics_fun/DC_motors

Issue Summary

Originally posted as part of #127

You can get the error get_pref': undefined method []’ for nil:NilClass` when trying to launch the CI script.

$ bundle exec arduino_ci_remote.rb
Attempting to download Arduino package with open-uri...[note: elipses elided]...
Extracting archive with tar
Located Arduino binary...                           /root/arduino_ci_ide/arduino
/var/lib/gems/2.5.0/gems/arduino_ci-0.2.0/lib/arduino_ci/arduino_cmd.rb:106:in `get_pref': undefined method `[]' for nil:NilClass (NoMethodError)
	from /var/lib/gems/2.5.0/gems/arduino_ci-0.2.0/lib/arduino_ci/arduino_cmd.rb:78:in `lib_dir'
	from /var/lib/gems/2.5.0/gems/arduino_ci-0.2.0/exe/arduino_ci_remote.rb:176:in `perform_unit_tests'
	from /var/lib/gems/2.5.0/gems/arduino_ci-0.2.0/exe/arduino_ci_remote.rb:383:in `<top (required)>'
	from /usr/local/bin/arduino_ci_remote.rb:23:in `load'
	from /usr/local/bin/arduino_ci_remote.rb:23:in `<main>'

This happens when arduino --get-pref fails, as no settings are parsed.

$ /root/arduino_ci_ide/arduino --get-pref
Picked up JAVA_TOOL_OPTIONS: 
Loading configuration...
Initializing packages...
cc.arduino.contributions.SignatureVerificationFailedException: package_index.json file signature verification failed
	at cc.arduino.contributions.packages.ContributionsIndexer.parseIndex(ContributionsIndexer.java:91)
	at processing.app.BaseNoGui.initPackages(BaseNoGui.java:483)
	at processing.app.Base.<init>(Base.java:262)
	at processing.app.Base.main(Base.java:145)

Expected behavior

The code should react immediately to the failure of the arduino binary, and print a helpful error message. It should also raise some form of exception and abort (as nothing can be salvaged).