[Feature request] Neon tells the user when something is wrong with the uploaded mei file

We've been having difficulties with mei files in Neon recently; the files are either old (like for Liber) or very recently produced by Mothra, and they behave badly in Neon. Sometimes the image and mei don't load, or they do load but certain user actions either don't work or don't work as expected. It would be very useful if Neon could alert the user to any abnormalities found in the mei file, so that we could figure out whether the buggy behaviour is due to the mei file, or to a problem within Neon, and fix it!

See #1378 and #1364.