example.ts

import { _throw } from 'rxjs/observable/throw';
import { of } from 'rxjs/observable/of';
import { Actions } from '@ngrx/effects';

class MyEffects {
  @Effect() doSomething$ = this.actions$
    .ofType('SOMETHING')
    .mergeMap(action => {
      return this.http.post('/endpoint')
        .mergeMap(result => {
          if (result.type === BAD) {
            return _throw(result); 
          }
        
          return of(result);
        });
    })
    .map(() => { /* to a success action */ })
    .catch(error => { /* to an error action */ });
}