Add explicit exception and documentation for when types are valid

[tajobe]

References #15

As per #15, choosing an invalid type for given configuration used to result in an unclear ClassCastException:

java.lang.ClassCastException: org.simplemc.simplehealthbars2.healthbar.NameHealthbar cannot be cast to org.simplemc.simplehealthbars2.healthbar.PlayerHealthbar

Now, you'll end up with:

java.lang.IllegalStateException: Invalid player healthbar type! Must be one of: SCOREBOARD, ACTION

or

java.lang.IllegalStateException: Invalid mob healthbar type! Must be one of: NAME, ACTION

Ideally, these would come from code rather than being specified in the string literal, but that'll require some reorganization.