don't report firewall isup as error in the logs

Currently we get a continuous stream in the logs of:

2019/09/05 14:18:21 Error while running bitmask-root:
2019/09/05 14:18:21 args:  [/snap/bin/riseup-vpn.bitmask-root firewall isup]
2019/09/05 14:18:21 output:  bitmask-root: INFO: bitmask firewall is down

Because bitmask-root answers with the return code to signal if is up or not, and bitmask-vpn uses the return code to signal if there was an error or not executing bitmask-root.