Elevate interpolation errors to WARNING log-level

[madig]

I stumbled over this while fontmaking:

[...]
INFO:mutatorMath:Some-Font.ufo:
Errors calculating 2 glyphs:
        ae
        ampersand
[...]

Shouldn't this be at least a WARNING? If so, I can provide a PR.

[LettError]

Please have a look at UFOProcessor. While it might not solve this exact problem, I'm taking the UFO generating out of mutatorMath. I'll gladly fix it there.