adding testfont seanjacob.ufo with letters a e n o s
Replace KeyError with CPSFormulaError

KeyError was a poor choice in this context, because dictionary lookups
are controlled partly by listening for KeyError.
Also, KeyError was not imported by the module (see #219)
Used a wrong name to create the error message. #219
load all masters before export (fixes #219)
Merge pull request #218 from rrthomas/interpolate

Fix new interpolate command