Fix behaviour of relation-get hook tool when no --app flag passed

[Aflynn50]

The relation-get hook tool defaults to getting info about the unit relation. It has an --app flag to get information about the application relation instead.

The documentation for the function that determins which relation to get (determineUnitOrAppName) states that if no --app flag is provided and an app name is passed then an error should be thrown. This is not what happens.

I have added in an error that is returned in this case, and an extra check to see if it is a valid unit name or not (i.e. it could be an invalid unit name and application name). I have updated the unit tests to reflect this and removed a test confirming the erroneous behavior.

Checklist

• Code style: imports ordered, good names, simple structure, etc
• Comments saying why design decisions were made
• Go unit tests, with comments saying what you're testing
  - [ ] Integration tests, with comments saying what you're testing
  - [ ] doc.go added or updated in changed packages

QA steps

Unit tests

go test ./worker/uniter/runner/jujuc

Black box test

# Setup
juju bootstrap lxd lxd-dev
juju add-model default
juju deploy ./testcharms/charms/dummy-sink
juju deploy ./testcharms/charms/dummy-source
# Wait for machines
juju integrate dummy-sink dummy-source

# Test app name with no --app flag
juju exec --unit dummy-sink/0 "relation-get -r 0 - dummy-sink"   
# ERROR expected unit name, got applicaiton name dummy-sink

# Test invalid unit name
juju exec --unit dummy-sink/0 "relation-get -r 0 - dummy-sink//0"
# ERROR invalid unit name dummy-sink//0

Links

Launchpad bug: https://bugs.launchpad.net/juju/+bug/2053282

[anvial]

I'm OK with code, and waiting simple.canonical.com to return back for QA.

[anvial]

LG2M

[Aflynn50]

/merge

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@hpidcock @anvial