title: How to reject a promise from inside then function
body: <p>This is probably a silly question, but mid promise chain, how do you reject a promise from inside one of the then functions?  For example:</p>
  
  <pre><code>someActionThatReturnsAPromise()
      .then(function(resource) {
          return modifyResource(resource)
      })
      .then(function(modifiedResource) {
          if (!isValid(modifiedResource)) {
              var validationError = getValidationError(modifiedResource);
              // fail promise with validationError
          }
      })
      .catch(function() {
          // oh noes
      });
  </code></pre>
  
  <p>There's no longer a reference to the original resolve/reject function or the PromiseResolver.  Am I just supposed to add <code>return Promise.reject(validationError);</code> ?</p>