Make docker build return exit code of build step

If a command during build fails, `docker build` now returns with the exit code of that command.

This closes #1150.


It seems like either tests or the buildbot is already broken for current master: http://docker-ci.dotcloud.com/builders/docker/builds/418

I can't get all the tests to pass either (neither directly on my ubuntu 13.04, in the docker dev env, in vagrant on ubuntu 12.04 and in the docker dev env within vagrant), there is still this test failing:

--- FAIL: TestPrivilegedCanMount (0.84 seconds)
        container_test.go:1508: Could not mount into privileged container

But that's the case for both master and this branch, so I'm confident this shouldn't introduce regressions.