Fixes for Github status reporting

Firstly there was a tiny syntax error causing `updateGitHubStatus()` to always return prematurely which prevented statuses from ever being reported.

The second commit defers updating the status rather than explicitly calling it at the end of `execute()`. This is a personal preference so let me know if you'd like to change it. Due to some weirdness in my dev environment I was seeing builds fail much earlier so it seems to make sense that an error in Drone is a valid reason to report a build error on the pull request (or at least call out that it couldn't be built).