webservicex.net currency converter can return -1 in case of its internal failure (I saw it on 2016-01-01 while converting USD => EUR) but Magento 2 does not check for the -1 result

[dmitrii-fediuk]

https://mage2.pro/t/374

[arkadiych]

Internal ticket MAGETWO-52780

[cspruiell]

This issue has been fixed and delivered to the develop branch.