_getById handle unknown keys better

[vsavkin]

Right now when invoked with an unknown key, _getById will throw new NoProviderError(null), which is not very helpful.

_getById(int keyId) {
  //switch ....
  throw new NoProviderError(DirectiveInjector._KEYS[keyId]);
}

This can be improved by changing NoProviderError to include the keyId in addition to the key itself.