return exit codes

[facundominguez]

Currently, sparkle arranges for main to be called but it ignores the returned exit code. This value should be somehow propagated to Java to give it an opportunity to do proper clean up.

The main use case is cleaning up temporary files produced by spark when there are test failures in test suites that rehearse sparkle.

Related to this: it has been noted elsewhere that the main() function that sparkle calls during bootstrap never returns control to the caller.

[facundominguez]

Probably, it should be done by throwing a Java exception before returning control to the caller.

[facundominguez]

Propagating exceptions to spark has been implemented in #154