Server returns HTTP 500 on submission when no reviewers exist

[lberrymage]

When no reviewers are configured in the backend's database, the code which randomly selects a reviewer fails, causing the server to return an HTTP 500 error when submitting a new app.

This isn't a major issue since it only occurs when there are no reviewers (which isn't the case in our production environment), but it's annoying to encounter in development environments, so we should return a proper error in the API (and perhaps modify the UI as well) to clearly indicate when there are no reviewers available to assign.