RegionServer could get stuck during shutdown process

When the server is being aborted or stopped, the server waits for all online regions to flush and close. And if a region is at the end of opening process, the openEventHandler throws an exception. However, it fails to remove that region from the on-line regions and fails to notify the master. This would prevent the server shutdown loop to exit.

Similarly, if regions that are being closed fail for any reason, the closeEventHandler throws, but again, it fails to remove that region from the on-line list.