Better error message for missing half of an adapter

[JakeWharton]

Given

class AnInterfaceAdapter {
  @ToJson String to(AnInterface ai) {
    return "an_interface";
  }
}

Moshi moshi = new Moshi.Builder()
    .add(new AnInterfaceAdapter())
    .build();
moshi.adapter(AnInterface.class);

An error message is thrown saying no adapter could be found for AnInterface. This is not strictly true and misleading, the problem is that only a "to" adapter could be found but no "from" adapter. We should report a better error message.