If we throw, we either have to catch the exception and turn it back into a dialog, or sacrifice the UX somewhat (produce a response which says "Exception!" and doesn't have a title), since we don't currently have a general-purpose way to throw an exception which has "here's how to render a nice dialog out of this" information.

It's easier to just throw a little deeper, but I'm trying to make this a little nicer if users fiddle with contextPHID for some reason, since it's technically user-editable.

Possibly we should have a user-facing sort of exception (and PhortuneDisplayException is a small step toward this), but then the signature needs to be setContextObject($context, $cancel_uri), which seems unintuitive.

Maybe this should be:

try {
  $this->setContextObject();
} catch (PhabricatorUserFacingException $ex) {
  $ex->setCancelURI($cancel_uri);
  throw $ex;
}

...with:

$developer_message = pht('contextPHID points at an invalid object');
$user_message = pht("oopsie woopsie OwO that's not a valid context ;)");
  
throw new id(new PhabricatorUserFacingException($developer_message))
  ->setTitle(pht('Bad Context Object'))
  ->setMessage($user_message)

...but that sure feels like a lot of work and very easy to get wrong.