Redirect to ManuscriptListView when manuscript is not found

[dchiller]

Overrides the default retrieve method of the ManuscriptDetailView class so that when a manuscript with the requested ID is not found, a custom 404 page is returned that directs users to the ManuscriptListView (see below).

As mentioned in the function's comment, this is meant to mitigate cases where a link to a specific manuscript was published before manuscripts in Cantus Ultimus had stable ID's.

Closes #802

Custom 404 Page https://user-images.githubusercontent.com/11023634/284639619-89ed0747-a41a-46ae-82f8-a69cdda4b741.png

[ahankinson]

Why not do both? Return a 404 (signalling to browsers, caches, search engines, etc. that it's no longer there) but with a custom 404 page pointing to the MSS list, and letting the users know that they should update their bookmarks.

[ahankinson]

Further reading: https://www.hallaminternet.com/what-are-soft-404-errors-will-they-affect-rankings/

[dchiller]

Ok, this makes sense! thank you

[dchiller]