issues and/or stats: a list of possible reasons why there are no returned units: NO_ACTIVE_CAMPAIGNS, CAMPAIGNS_NOT_SOUND, NO_DEPOSITASSET_CAMPAIGNS, NO_UNITS_FOR_SIZE, NO_UNITS_FOR_TARGETING, NO_UNITS_FOR_ADSLOTRULES, SLOT_NOT_VERIFIED (if acceptedReferrers.length == 0)
Enhancement from https://github.com/AdExNetwork/adex-market/issues/101