[makeotfexe] Misleading error for lookup blocks before languagesystem

[khaledhosny]

The following file causes makeotfexe to fail with:

makeotfexe [WARNING] <font> [internal] Feature block seen before any language system statement. You should place languagesystem statements before any feature definition [a.fea 1]
makeotfexe [ERROR] <font> languagesystem must be specified before all feature blocks [a.fea 5]
makeotfexe [FATAL] <font> aborting because of errors

lookup a {
    sub a by a;
} a;

languagesystem latn dflt;

feature liga {
    lookup a;
} liga;

But there are no features before the languagesystem statement so the message is misleading. The spec only mentions feature blocks too. So either the spec or the code need to be updated to match the other.