Remove use of System.exit() calls from codebase

[lewismc]

I'm currently working on building openie into Apache Any23 via apache/any23#33, a major issue I am having however is with Unit testing the code I'm attempting to implement.
The is due to the Maven surefire plugin failing whenever it encounters System.exit().
I've scanned the openie codebase and there appears to be a few places where this is the case.
I am not 100% sure that this stems from openie, however I will begin by submitting the PR here to narrow the issue down.

[lewismc]

@Yongyao CC for context

[jkinkead]

The Relnoun case is definitely an error; that should be throwing an exception or similar.

The Nesty case is in a main method, so it seems appropriate.

[lewismc]

OK thanks @jkinkead I'm working on a PR and testing locally to see if this resolves my issue.

The Nesty case is in a main method, so it seems appropriate.

Two things spring to mind

• is there any reason for returning the non-zero code?
• Seeing as we are within main this would be all the more reason to not use System.exit(). Main will terminate after the exception message has been thrown.

[aimichal]

We're not actively working on making changes to this codebase at the moment. If you have a suggestion for what to change, a PR would be welcome. (I agree that deeply nested library code shouldn't make System.exit() calls.)

Closing this issue now as part of cleanup, but please re-open if there's more to discuss.