code: protected static <T> Action1<Throwable> onErrorFrom(final Observer<T> observer) {
          return new Action1<Throwable>() {
              @Override
              public void call(Throwable t1) {
                  observer.onError(t1);
              }
          };
      }
docs: Extracts a method reference to the Observer's {@link Observer#onError(java.lang.Throwable) onError}
  method in the form of an {@link Action1}.
  <p>Java 8: observer::onError</p>
  
  @param observer
  the {@link Observer} to use
  @return an action which calls observer's {@code onError} method.