Startable's close method may have Exception signature

[PMacho]

At the moment the Startable classes close() method looked like so:

@Override default void close() { stop(); }

However, Startable implements AutoClosable where the close method has an Exception signature. I suggest to keep this signature in the close() method of Starable.

Reason: It allows users to override the close method, executing other methods that might also throw exception, without explicitly catching them.

[bsideup]

This is done for convenience so that the consumers don't need to catch the exception, especially if they use try-with-resources. We generally prefer making this easier at the consumption side, not implementation, and a single try-catch in close() is much better than many catches at use use :)