Add garden.ProcessNotFoundError error type

[vito]

Now that process IDs can be injected via (Container).Run, I'd like to have (Container).Attach return this specific error so that I can check for it and know to do (Container).Run with the same ID instead. This would allow Concourse to stop tracking this state as properties on the container, and make the reentrancy a lot simpler and less error-prone.

This should also not be logged at an error level, just so operators don't see the noise; it's "happy path" to optimistically look for a process to be already running, and would happen multiple times for every build (once per step).

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/144105141

The labels on this github issue will be updated when the story is started.

[MissingRoberto]

@vito Garden has being extended to return a new error ProcessNotFoundError when the requested process doesn't exist.

[vito]

Yay!