put up message about missing images

[annamorphism]

Until we fix the issues with Salzinnes, MS 73, etc (#933, #922) we need to have a message to alert people that their images won't load, so they aren't stuck there forever...

[annamorphism]

note: B-DEa 9 has the same issue, for I assume the same reason.
F-Pn Lat. 15181 (https://cantus.simssa.ca/manuscript/123631/ gives a message (coming from Gallica, I assume):

[kyrieb-ekat]

Gallica went down for me yesterday, they might still be down from whatever that is. This could be a subissue for a large thing, wherein currently everything we're talking about (Salzinnes) is because we're running prezi2 (and I pumped the brakes on updating to diva6 because we swiftly got diva7! which needs to be completed urgently). The other problem is a checker for when its upstream of us...

[kyrieb-ekat]

MS73 does load, but it's via Internet Archive, so it genuinely just takes forever. I have been exploring side options for hosting it somewhat passively, I'll kick that up to a higher level of urgency...