refactor: improved error handling

[talentedmrjones]

Added

• new test tests/fail-bad-url.yml

Changed

• Error handling in general.
  • engine/config.go LoadConfig was exiting directly when the caller was also configured to exit based on the returned error. LoadConfig now just reports the error.
  • engine/test.go was exiting directly, now just reports to engine.Run which reports to root command.
• root.go
  • fixed cli name
  • now exiting based on result of engine.Run
• engine/test.go refactored logic of test error handling and reporting

[devon-berta-aquia]

LGTM