In general, the new "New Review Request" page works very well for people. One

feature that was missed by a small number of users was the ability to specify a

parent diff. This change adds back that feature.
If the given diff fails due to missing files or revisions, the user is shown

the error, but additionally given the option of uploading a parent diff. This

keeps the streamlined workflow for the general case but makes it possible for

the complex one.
While I was in here, I also plumbed through errors when creating the review

request or diff to display. I don't think we've ever hit these errors in

practice, due to the diff validation step (I had to artificially hack the code

to trigger them).

Uploaded a plain diff via both DnD and the select button and saw it create

  the review request.
Uploaded a diff which required a parent diff and saw the expected error

  message with new form.
Uploaded diff + parent diff using both DnD and the select button, and saw it

  create the review request.
Uploaded a broken diff and saw the right error.
Uploaded a diff and parent diff which still required another parent to

  completely resolve all changes and saw an error.
Checked that "Start over" worked on both error pages.
Ran jshint
Ran js-tests