Exit if dyndns not set

[pablomendezroyo]

If dyndns is not propertly get from the dappmanager API, the wireguard app should exit with an error so the container keep restarting until it gets the dynds.

TODO

• At the script 30-config add something similar to

  # Exit if SERVERURL not set
  [ -z "$SERVERURL" ] && { echo "**** ERROR: External server address not found at my.dappnode/global-envs/HOSTNAME ****"; exit 1}

• Add env var to exit on error in dockerfile S6_BEHAVIOUR_IF_STAGE2_FAILS increase delay if dyndns not set #19 (comment)