Add Errno::EHOSTUNREACH to connection errors

[pelargir]

This adds Errno::EHOSTUNREACH to the list of default connection errors. This exception is occasionally raised from the Net::HTTP library and should be handled the same way as other connection errors. (This exception is already being handled in ActiveMerchant.)

[pelargir]

I signed the CLA but I don't see a way to re-run the check.