The corpus details page stays in a loading state when a corpus is not found

1. Open the project information for any project.
2. Change the UUID in the URL so that it points to a project that does not exist, and press Enter.
3. The project form stays in a loading state, but you can move around the tabs.
4. Click on the Allowed metadata or Entity types tab to reproduce #1226 (closed).
5. Cry.

We should use the corpora store's corporaLoaded property to tell when the corpora are available, and show the usual yellow warning saying the project does not exist.