juju-run gives a poor error when the unit is ambiguous

[James Troup]

juju-run --help claims that you can pass an empty string to -u, but this doesn't appear to be true.

| root@grafana-1:~# juju-run --help

[...]

| If the there's one and only one unit on this host, <unit-name>
| is automatically inferred and the positional argument is not needed.
| If -u is passed an empty string, this behaviour is also observed.
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
| Examples:
| juju-run app/0 hostname -f
| juju-run --no-context -- hostname -f
| juju-run "hostname -f"
| juju-run -u "" -- hostname -f
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| juju-run -u app/0 "hostname -f"
| juju-run -u app/0 -- hostname -f
|
| The commands are executed with '/bin/bash -s', and the output returned.
| root@grafana-1:~# juju-run -u "" -- hostname -f
| ERROR no unit
| root@grafana-1:~#

[Joseph Phillips (manadart)]

This appears to be the help for a plugin. I've marked it invalid for the Juju project.

[John A Meinel (jameinel)]

This isn't a plugin, this is /usr/bin/juju-run which we create for things like Cron to be able to create a unit hook context and run a script as that unit.
eg:
```
$ juju ssh 0
ubuntu@juju-883c05-0:~$ juju-run --help
Usage: juju-run [options] [-u] [<unit-name>] <commands>

Summary:
run commands in a unit's hook context

Options:
--force-remote-unit (= false)
    run the commands for a specific relation context, bypassing the remote unit check
```

Which has both:
```
-u (= "-")
    explicit unit-name, all other arguments are commands. if -u is passed an empty string, unit-name is inferred from state
```
and
```
If the there's one and only one unit on this host, <unit-name>
is automatically inferred and the positional argument is not needed.
If -u is passed an empty string, this behaviour is also observed.
```

Note, though, that on my test instance it does work:
root@juju-883c05-0:~# juju-run -u "" -- echo hello
hello

But note the caveat: *If there is one and only one unit*.

If you are on a machine with subordinates, etc, then it cannot detect which unit you want, because it is ambiguous.

[John A Meinel (jameinel)]

I think the actual fix is that: "ERROR no unit" is a bad error message, and should instead be something like: "ERROR unit is ambiguous, could be one of 'a', 'b', 'c'