Common Exception throwing

[zsimjee]

1. Malformed YAML. Let's find a way to throw a known exception on that and show it in the frontend. Also address the ops-frontend change(s) required to get that passed all the way up
2. Misssing dependency. If a widget's dependency is not in the dep graph, it currently kills the page. Let's make that a context-specific error and throw on that. Likewise for the frontend.