Fix error on explorer distribution update when distribution does not exist

When a distribution has already been deleted, trying to update it or delete it again will lead to a 500 error. It should show a 404 page instead.

Steps to reproduce: call update or delete API with a doc id that does not exist on the repository:

• http://localhost:8080/nuxeo/site/distribution/updateDistrib/server-11.3-SNAPSHOT?distribDocId=b60794c1-eeda-4036-bafd-0dedb77b4baa
• http://localhost:8080/nuxeo/site/distribution/delete/server-11.3-SNAPSHOT?distribDocId=b60794c1-eeda-4036-bafd-0dedb77b4baa