Taskrunner silently fails when tasks aren't defined in workspace.taskrunner.json

[anubhav94]

If you delete a task from tasks.go but still have it defined in workspace.taskrunner.json, taskrunner silently fails without any output.

Relevant lines that need to be changed (From Stephen): https://github.com/samsarahq/taskrunner/blob/master/runner.go#L109-L112