False positives on non-existent sessions in `/ma delete-session-stats`

[garbagemule]

If you pass a valid, but non-existent, UUID to the delete-session-stats command, it will wrongly claim that the non-existent session was deleted. The plugin should inform the command sender that no such session exists.

Two potential solutions:

• Throw NoSuchElementException/IllegalArgumentException from the StatsStore implementation(s) and catch the exception in the command.
• Modify the StatsStore interface to return a boolean; true for success, false for failure. This should be possible on the JDBI side of things by just checking for the number of affected rows after statement execution.