Add explicit throw methods for getting index state and fields

[aprudhomme]

IndexState.getField(name), GlobalState.getIndex(name), and GlobalState.getIndexStateManager(name) all throw an IllegalArgumentException if the index/field does not exist. This makes it harder to handles cases where something not existing is acceptable.

I modified these methods to return null instead of throwing an exception. Separate methods have been added (*OrThrow()) that preserve the status quo behavior. Almost all existing usage of these methods has been change to use the throwing version. The DocLookup in the IndexState uses the non-throwing version, as this provides consistent behavior with the lookup created during search request processing.

None of the currently ported plugins use this api, so they should still be compatible.

The example-plugin test is expected to fail until it gets updated for the alpha.2 release.