frontends: use new EXIT_BACKEND_NOT_FOUND exit code: - new EXIT_BACKEND_NOT_FOUND (5) is used when frontend can't find backend with the bridge. It differs from EXIT_BRIDGE_ERROR (3) which is used when something went wrong when connecting to backend. - use this exit code in quick app and in jp

version: "3.6"
services:

  sat:
    image: salutatoi/sat
    build: backend

  libervia:
    image: salutatoi/libervia
    build: libervia
    depends_on:
      - sat