dcos-net: dcos_dns_handler crashes if upstreams are not available

2018-05-22 03:04:30 =CRASH REPORT====
  crasher:
    initial call: dcos_dns_handler:init/4
    pid: <0.2428.0>
    registered_name: []
    exception error: {function_clause,[{lists,map,[#Fun<dcos_dns_handler.3.14594562>,{{10,0,2,150},61053}],[{file,"lists.erl"},{line,1238}]},{lists,map,2,[{file,"lists.erl"},{line,1239}]},{dcos_dns_handler,resolve,4,[{file,"/pkg/src/dcos-net/_build/prod/lib/dcos_dns/src/dcos_dns_handler.erl"},{line,117}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]}
    ancestors: [dcos_dns_handler_sj_4,dcos_dns_handler_sj_worker_sup,dcos_dns_handler_sj,sidejob_sup,<0.1202.0>]
    message_queue_len: 0
    messages: []
    links: [<0.1258.0>]
    dictionary: []
    trap_exit: false
    status: running
    heap_size: 610
    stack_size: 27
    reductions: 464
  neighbours:
crasher:
    initial call: dcos_dns_handler:-start_worker/3-fun-0-/0
    pid: <0.2429.0>
    registered_name: []
    exception exit: {noproc,[{dcos_dns_handler,udp_worker,3,[{file,"/pkg/src/dcos-net/_build/prod/lib/dcos_dns/src/dcos_dns_handler.erl"},{line,178}]},{proc_lib,init_p,3,[{file,"proc_lib.erl"},{line,232}]}]}
    ancestors: [<0.2428.0>,dcos_dns_handler_sj_4,dcos_dns_handler_sj_worker_sup,dcos_dns_handler_sj,sidejob_sup,<0.1202.0>]
    message_queue_len: 0
    messages: []
    links: []
    dictionary: []
    trap_exit: false
    status: running
    heap_size: 610
    stack_size: 27
    reductions: 864
  neighbours: