title: Rx: How to buffer items until condition is met
body: <p>I have a stream of items and I want to buffer them until one of the items meets a condition. Once the condition is met, the buffered items should be delivered to the subscriber.</p>
  
  <p>If the source observable completes but the condition hasn't been met yet, I want an error to be produced.</p>
  
  <p>Can this be achieved using default operators in RxJava or another language of the Rx family?</p>