toJSON should throw

The toJSON methods in Array, Dictionary, Result and Document should all throw a checked exception if they fail because some string cannot be translated from UTF-8 (Fleece) encoding, to UTF-16 (Java String encoding)

Currently all except Document are annotated as @NonNull, when, in fact, they can return null. Document is annotated as @Nullable. All should be annotated @NonNull (not a breaking change) but changed to throw a CouchbaseLiteException (API breaking change)