title: How to map error from sendError: on RACSignal
body: <p>This is very cumbersome</p>
  
  <pre><code>RACSignal *originalSignal = ...;
  
  RACSignal *mappedSignal = [[[[[originalSignal publish] autoconnect] materialize] map:^id(RACEvent *event) {
      if(event.eventType == RACEventTypeError)
      {
          return [RACEvent eventWithError:[self errorForHelperError:event.error]];
      }
      return event;
  }] dematerialize];
  </code></pre>