Font Library: show allowed upload format and display errors for incorrect uploads

[annezazu]

Pulling this feedback from the FSE Outreach Program for consideration for stage 3 of the Font Library work:

I tried to upload a .zip on purpose but got no errors. It may be nice to have or suggest allowed font formats and a confirmation notification when one gets uploaded.

[annezazu]

Another piece of feedback on this note from the same call for testing:

Went to fonts.google.com and downloaded Fuggles.zip. It looks like I have to unzip the folder and then upload the Fuggles-Regular.ttf font file.

[Mamaduka]

Thre's already an issue for improving the error messages - #54777.

[annezazu]

I saw this as slightly different but, for consolidations' sake, I'll close and add the feedback there :)