Generate _main.ks

To generate _main.ks, you just need to run |make certs| (keytool and mekeytool should be in your path).
|make certs| isn't run by default, because we don't want to re-generate _main.ks for each build but only when we want to add new certs (this allows people to build without keytool and mekeytool).
Hmm, since the number of keys can change from build to build, I don't think we should assume that the store will have a specific number of keys. It's reasonable to assume it'll have at least one key, though, since users who don't rebuild it will get the default store, while those who do should at least import the GoDaddy key into it.

So I would only check that the store has at least one key.