worker/envworkermanager: always stop worker on exit

[rogpeppe]

This fixes an intermittent test failure in TestLoopExitKillsRunner
which happened because the underlying worker was not killed
when the loop terminated with an error.

We also change the API into a slightly more general form - there's
no need for the EnvWorkerManager to know that the worker
it's dealing with is actually a worker.Runner, and this makes
the tests slightly simpler to write (and arguably the API easier
to understand)

(Review request: http://reviews.vapour.ws/r/1858/)

[rogpeppe]

$$merge$$

[jujubot]

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