Better error message when loading a non-manifest

[mk-pmb]

Manifest URLs: (Edit: None.)

I accidentially gave an AnnotationPage as the manifest URL. The error message was just "TypeError: n is null", which had me confused for a few minutes.

Can we get a better error message for when the thing at the manifest URL is not a manifest?

[lutzhelm]

PR #3927 already addresses handling the JavaScript error itself. It would currently show the following already existing error message:

The resource cannot be added: https://gist.githubusercontent.com/mk-pmb/4b73909b15a501344675240221eb2f05/raw/1ff0564fd1c52783eb34bb19cca3012b27ce6656/mirador-null.iiif3.json

Would this be sufficient for you? Announcing the specific cause of the error (not a manifest or collection; manifest / collection has errors; parser handles iiif resource wrong etc.) might prove difficult.

[mk-pmb]

I think I still wouldn't have understood my mistake immediately. Can we maybe report whether the @context and type had their expected values? Then it could say something like

The resource cannot be added: [… URL …] The @context field was understood. The type field was unsupported (expected "Manifest").