Add Core#isOpen public method, add CoreClosedException

[chrislo27]

The open AtomicBoolean field in Core is not publicly gettable, so I added a boolean getter Core#isOpen(). Additionally, I subclassed IllegalStateException and created CoreClosedException which is easier to catch for than a plain IllegalStateException.

My rationale for this change is that any of the SDK methods that eventually call (the package-private) Core#execute method may throw an IllegalArgumentException if the Core is unexpectedly closed. I would prefer to avoid exception-based handling as a user of this library, so being able to check if the Core is open or not is useful. The CoreClosedException was my original plan since I could catch-filter for it, but I think both options are flexible.

[JnCrMx]

Both changes look very useful and good to me. Thank you a lot for your contribution! :D