[ds5] Freeze if fontTools error

[ryanbugden]

1. Add input that will yield an error, such as fontTools.varLib.errors.VarLibValidationError.
2. Hit Generate.

Error show sin Output Window. Now RF has to restart.

[ryanbugden]

Same with this example from a designspace.
fontTools.ufoLib.errors.UFOLibError: The glyph "B" occurs in too many kerning groups.

I think the progress window should probably at least close if there's an error. Pop-up error message would be better.