daemon/prometheus: fix crash if listener cannot be created
daemon/prometheus: fix crash if listener cannot be created

refs #238

  zrepl version=v0.2.0-11-gdc39c81 GOOS=linux GOARCH=amd64 Compiler=gc
 starting daemon
 [pull_source]: starting job
 [_prometheus]: starting job
 [connection_loss_tidyup]: starting job
 [connection_loss_tidyup]: wait for wakeups
 [_control]: starting job
 [_prometheus]: cannot listen err="listen tcp 10.0.0.200:9091: bind: cannot assign requested add
 [_prometheus]: job exited
 panic: runtime error: invalid memory address or nil pointer dereference
         panic: runtime error: invalid memory address or nil pointer dereference
 [signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x81ea4d]
 goroutine 25 [running]:
 net/http.(*onceCloseListener).close(...)
         /usr/local/go/src/net/http/server.go:3330
 sync.(*Once).doSlow(0xc00018b060, 0xc0000c7bc0)
         /usr/local/go/src/sync/once.go:66 +0xe3
 sync.(*Once).Do(...)
         /usr/local/go/src/sync/once.go:57
 net/http.(*onceCloseListener).Close(0xc00018b050, 0xc0003a6000, 0xe0)
         /usr/local/go/src/net/http/server.go:3326 +0x77
 panic(0xb1d1c0, 0x11d7d90)
         /usr/local/go/src/runtime/panic.go:679 +0x1b2
 net/http.(*onceCloseListener).Accept(0xc00018b050, 0xc000120020, 0xb0fd20, 0x11d7ce0, 0xbee6e0)
         <autogenerated>:1 +0x32
 net/http.(*Server).Serve(0xc0003a6000, 0x0, 0x0, 0x0, 0x0)
         /usr/local/go/src/net/http/server.go:2896 +0x286
 net/http.Serve(...)
         /usr/local/go/src/net/http/server.go:2468
 github.com/zrepl/zrepl/daemon.(*prometheusJob).Run(0xc000109940, 0xd19ec0, 0xc00018a930)
         /go/src/github.com/zrepl/zrepl/daemon/prometheus.go:75 +0x23e
 github.com/zrepl/zrepl/daemon.(*jobs).start.func1(0xc0000f68c0, 0xd22c40, 0xc000116ee0, 0xd1ba4
         /go/src/github.com/zrepl/zrepl/daemon/daemon.go:220 +0x121
 created by github.com/zrepl/zrepl/daemon.(*jobs).start
         /go/src/github.com/zrepl/zrepl/daemon/daemon.go:216 +0x52e
 zrepl.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
 zrepl.service: Failed with result 'exit-code'.

zrepl/zrepl,zrepl/zrepl