cli: Make "unknown sub-command" error clearer for empty subcommands

[a-robinson]

I ran cockroach zone to try and get the list of available sub-commands
printed as help text, but ended up getting confused by what it printed:

Error: unknown sub-command:
Failed running "zone"

I wasn't really sure what this meant. Now it'll print:

Error: unknown sub-command: ""
Failed running "zone"

It may also be better to say something like "unknown or missing
sub-command". Let me know if you'd prefer that.

Release note: None

---

Along these lines, how come the available subcommands don't get printed anymore? I can't get them listed either with cockroach zone or cockroach zone -h. I could have sworn we used to list them. Did cobra change? Maybe I'm conflating cockroach with other tools. It'd certainly be nice if we did, though.

[cockroach-teamcity]

This change is 

[knz]

What you have now LGTM

The other suggestion LGTM too!

Pick your preferred outcome. Either is better than the status quo. Thanks for finding this out.

[a-robinson]

Thanks for the quick review. Also, I double checked, and not printing the subcommands is a regression since 2.0. I'll open a separate issue, but do you have any available time to look into it?

~ 0 $ Downloads/cockroach-v2.0.3.darwin-10.9-amd64/cockroach zone
Usage:
  cockroach zone [flags]
  cockroach zone [command]

Available Commands:
  get         fetches and displays the zone config
  ls          list all zone configs
  rm          remove a zone config
  set         create or update zone config for object ID

Flags:
  -h, --help   help for zone

Global Flags:
      --log-backtrace-at traceLocation   when logging hits line file:N, emit a stack trace (default :0)
      --log-dir string                   if non-empty, write log files in this directory
      --log-dir-max-size bytes           maximum combined size of all log files (default 100 MiB)
      --log-file-max-size bytes          maximum size of each log file (default 10 MiB)
      --log-file-verbosity Severity      minimum verbosity of messages written to the log file (default INFO)
      --logtostderr Severity[=DEFAULT]   logs at or above this threshold go to stderr (default NONE)
      --no-color                         disable standard error log colorization
      --verbosity level                  log level for V logs
      --vmodule moduleSpec               comma-separated list of pattern=N settings for file-filtered logging

Use "cockroach zone [command] --help" for more information about a command.
~ 0 $ Downloads/cockroach-v2.1.0-beta.20180924.darwin-10.9-amd64/cockroach zone
Usage:
  cockroach zone [flags]

Flags:
  -h, --help   help for zone

Global Flags:
      --log-dir string                   if non-empty, write log files in this directory
      --log-dir-max-size bytes           maximum combined size of all log files (default 100 MiB)
      --log-file-max-size bytes          maximum size of each log file (default 10 MiB)
      --log-file-verbosity Severity      minimum verbosity of messages written to the log file (default INFO)
      --logtostderr Severity[=DEFAULT]   logs at or above this threshold go to stderr (default NONE)
      --no-color                         disable standard error log colorization
      --vmodule moduleSpec               comma-separated list of pattern=N settings for file-filtered logging (significantly hurts performance)
Error: unknown sub-command:
Failed running "zone"
~ 1 $

[a-robinson]

bors r+

[craig]

Build succeeded

• GitHub CI (Cockroach)