Release state back to the pool on errors

[babbageclunk]

In the functions that do more checks after the StatePool.Get but return
the state (so can't use a deferred release because it should outlive the
scope), we need to make sure that the state is returned to the pool when
we return an error and a nil state.

Use the named return values + deferred if err != nil release trick (which
really needs a catchier name) when there's more than one error path.

Includes a drive-by fix for the charmsHandler where I was releasing the
state immediately rather than deferring it, and a formatting fix - it looks like
someone's git doesn't have the pre-push hook set up.

[babbageclunk]

!!build!!

[babbageclunk]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/9664

[babbageclunk]

Storing the state pointer in the return variable meant that an error return would set it to nil before running the defer (which tried to release it).

$$merge$$

[jujubot]

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