onError hooks functionality

[daffl]

This allows to register onError hooks the same way as before and after hooks:

const app = feathers().configure(hooks()).use('/dummy', {
  get() {
    return Promise.reject(new Error('Something went wrong'));
  }
});
const service = app.service('dummy');

service.onError(function(hook) {
  // hook.type === 'onError'
  // hook.error === Error('Something went wrong')
  throw new Error('This is another error');
});

service.get().catch(e => console.error(e));

Closes #83

[ekryski]

Awesome @daffl! Being super knit-picky but do you want to just call it error instead of onError so that it looks similar to before and after? I think service.error is nice.

Totes your call. Other than that . Stoked for this to land!

[marshallswain]

Since before, on, and after are all prepositions and convey location, I prefer it to be onError. My 2 bits.

[eddyystop]

Would we want to know where the error occurred: before hooks, operation itself, after hooks? Perhaps that depends on whether the DB remains updated if its the after hooks that throw.

[daffl]

I thought about that but realized that you can get the original hook via hook.error.hook. Maybe we should alias it to hook.source?

[eddyystop]

Something like errorSource may be more self-explanatory?

[ekryski]

I agree with @eddyystop unless it simply becomes hook.error.source.

I noticed that we probably want to add a test to ensure you can chain up multiple error hooks (I think that's what we had intended).