Reject fonts incompatible with Samsa (including UFO, TTX, CFF2) gracefully

[Lorp]

Sometimes users try TTX, UFO and OTF containing CFF2 data in Samsa. The app should fail gracefully and inform users what was wrong with the file.

[Lorp]

Users also expect WOFF2 fonts to work (#89), as they do in other font testing apps such as Wakamaifondue.