Return non-zero exit code when tests fail or error

[mainej]

Fixes https://github.com/cognitect-labs/test-runner/issues/9. Based on https://github.com/cognitect-labs/test-runner/pull/12, with the following differences:

• Returns non-zero exit code on either failure or error. This is what CI environments expect, typically.
• ~Ensures shutdown-agents runs before exiting.~
  • ~This commit avoids a problem with PR #12, which calls System/exit before the finally block. That approach skips the finally block (1), unless System/exit is configured to throw a SecurityException (2), which I don't believe is the default in Clojure.~

1) https://stackoverflow.com/questions/65035/does-finally-always-execute-in-java 2) https://stackoverflow.com/questions/1410951/how-does-javas-system-exit-work-with-try-catch-finally-blocks

[puredanger]

Looks good to me but I will leave this to @levand to merge

[levand]

If you call System/exit, I think a finally block will not be called at all.

As such, if we're always calling System/exit explicitly, we don't even need to bother about cleaning up agents. Cleaning up agents is only necessary to allow the JVM to terminate "naturally" after the -main function returns. So I think this can be simplified a bit by taking out the "finally" altogether.

[mainej]

I think the finally block is called, because it is structured to run before the System/exit. But it sounds reasonable that it doesn't need to be called if we're going to System/exit. I'll submit a revision with that change.