Fix ERROR on ligatures conditional

[felipesanches]

in a few rare occasions, we get an ERROR on the ligatures conditional (which is required in tests such as com.google.fonts/test/064, the caret positioning test). This happens on only 7 of the 2400 TTFs in Google Fonts collection:

The actual error is this:

The condition implementation fails at line 2291:
https://github.com/googlefonts/fontbakery/blob/527dda3e1e5b3364512852a6d63dd7745273a6cc/Lib/fontbakery/specifications/googlefonts.py#L2287-L2300

Interestingly, when trying to use ttx to inspect the file contents I get a similar error, which makes me believe this is a case of corrupted TTF files:

This may also be a fontTools bug. Can you confirm that, @behdad ?

[felipesanches]

Some example of offending files from the Google Fonts collection are:

• ofl/bhavuka/Bhavuka-Regular.ttf
• ofl/seoulhangang/SeoulHangang-Medium.ttf
• ofl/seoulhangang/SeoulHangang-Light.ttf

[davelab6]

Given the errors happen directly when TTX'ing the fonts, I propose to update the checker runner so that if fontTools fails to unpack a font, that is a FAIL of the check, not an ERROR, because its not a problem with our code, its a problem with the font that this check has uncovered.

I see in the runner's output the string, unpack requires - perhaps you can find that string in the error and if found, return a FAIL?