daemon: remove containerNotModifiedError

Removing this type, because:

- containerNotModifiedError is not an actual error, and abstracting it away
  was hiding some of these details. It also wasn't used as a sentinel error
  anywhere, so doesn't have to be its own type.
- Defining a type just to toggle the error-message between "not running"
  and "not stopped" felt a bit over-the-top, as each variant was only used once.
- So  "it only had one job", and it didn't even do that right; it produced
  capitalized error messages, which makes linters unhappy.

So, let's just inline what it does in the two places it was used.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/start.go

@@ -30,7 +30,7 @@ func validateState(ctr *container.Container) error {
 		// already in the desired state. It's implemented as an error
 		// to make the code calling this function terminate early (as
 		// no further processing is needed).
-		return containerNotModifiedError{running: true}
+		return errdefs.NotModified(errors.New("container is already running"))
 	}
 	if ctr.RemovalInProgress || ctr.Dead {
 		return errdefs.Conflict(errors.New("container is marked for removal and cannot be started"))