Please complete the following fields as applicable: **What version of the DMPRoadmap code are you running? (e.g. v2.2.0)** - `v4.2.0` **Expected behaviour:** - Assuming plan.id = 99999 doesn't exist in the db, then the url path `/plans/99999` should return the following:  - One might expect the same behaviour when `/plans/99999/export.pdf?export%5Bquestion_headings%5D=true` is requested **Actual behaviour:** - An `ActionController::UnknownFormat` error is encountered instead 