STORM-449: Updated ShellBolt to not exit when shutting down.

I modified the test that was having issues to run 100 times in a row.  Previously it would fail after 1 or 2 times through the loop.  Now it passes all of the times no problem.  I didn't include the test change because it is overkill here.
If this `Throwable` is an error, we would still want to log it somewhere as well as shut down.