relation-get should error if --app is missing and app supplied

[Ian Booth]

If relation-get is run like so:

relation-get -r 7 ingress traefik

it is supposed to error because traefik is an app name (not a unit) and --app wasn't supplied.

The code comments say so (see 1b)

// The logic is as follows:
// 1) If a user supplies a unit or app name, that overrides any default
// a) If they supply --app and a unit name, we turn that back into an application name
// b) note, if they *don't* supply --app, and they specify an app name, that should be an error
// 2) If no unit/app is supplied then we look at our context
// a) If --app is specified, then we use the context app
// b) If --app is not specified, but we don't have a context unit but do have a context app
// then we set --app, and set the target as the app
// c) If we have a context unit, then that is used

[Ian Booth (wallyworld)]

https://github.com/juju/juju/pull/16938