worker/machiner: ensure api errors set before worker starts.

[howbazaar]

It seems blindingly obvious once it is pointed out, the error returns from the mock API need to be set before the worker that uses that API is started.

There is a race between the worker starting the goroutine that calls the setup and handler methods of the machiner, and the test setting the API error return values. In almost all circumstances, the test wins the race, but sometimes the worker goroutine calls an API before the error returns are set. The default behaviour of the mock NextErr() method is to return nil if none have been set. This means that the final error that is set by the test is never returned.

Fixes http://pad.lv/1612744

[howbazaar]

QA

Unit tests pass in worker/machiner

[howbazaar]

$$merge$$

[jujubot]

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