[r=dave-cheney] Fix nil pointer error during test setup
[r=dave-cheney] Fix nil pointer error during test setup

Now a missing mongod returns a much improved error

ubuntu@winton-02:~/src/launchpad.net/juju-core$ go test ./worker/uniter
--- FAIL: TestPackage (0.00 seconds)
        mgo.go:211: exec: "mongod": executable file not found in $PATH
FAIL
FAIL launchpad.net/juju-core/worker/uniter 0.509s

https://codereview.appspot.com/85100044/
reedobrien/store