Panic and crash when supervisord collector enabled, yet supervisord not listening on 127.0.0.1:9001

Host operating system: output of uname -a

Linux example.com 4.14.26-54.32.amzn2.x86_64 #1 SMP Tue Mar 27 21:50:30 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

node_exporter version: output of node_exporter --version

node_exporter, version 0.16.0 (branch: HEAD, revision: d42bd70f4363dced6b77d8fc311ea57b63387e4f)
  build user:       root@a67a9bc13a69
  build date:       20180515-15:52:42
  go version:       go1.9.6

node_exporter command line flags

./node_exporter-0.16.0.linux-amd64/node_exporter --collector.textfile.directory=/etc/prometheus/node_exporter --collector.supervisord

Are you running node_exporter in Docker?

No.

What did you do that produced an error?

Starting the node_exporter with --collector.supervisord enabled while supervisord isn't listening on 127.0.0.1:9001.

What did you expect to see?

To see an error, but for node_exporter to keep on running.

What did you see instead?

node_exporter panicking with this output.

Reproduce the failure by starting node_exporter with --collector.supervisord while supervisord is not listening on 127.0.0.1:9001, and inspect the supervisord collector entry point and linked panic output. Done means node_exporter reports the connection error without panicking and continues running with the collector enabled.