api/uniter: validate unit name before relation-get

[davecheney]

The name of the unit passed in from the cli is not a string form of a tag so we have to validate that it actaully is a unit name (the id part of a unit tag).

[jameinel]

So I think this is correct from a case of "validate input before turning it into objects", and on the case of fixing 1.20 this seems perfectly ok.
It feels like we're missing a standard, unified way of turning user input (service, unit/0, 0, etc) into names.Tag objects so that we can hand them around and be sure they are safe.
If NewUnitTag isn't intended to be this (you mentioned it is intended only for testing code), then we need something like this.
So I'm LGTM on landing this because it does fix a serious issue, but it feels like we're going to be playing whack-a-mole unless we have a consistent way of validating our inputs.

[davecheney]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju