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/stop.go

@@ -26,7 +26,12 @@ func (daemon *Daemon) ContainerStop(ctx context.Context, name string, options co
 		return err
 	}
 	if !ctr.IsRunning() {
-		return containerNotModifiedError{}
+		// This is not an actual error, but produces a 304 "not modified"
+		// when returned through the API to indicates the container is
+		// 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 errdefs.NotModified(errors.New("container is already stopped"))
 	}
 	err = daemon.containerStop(ctx, ctr, options)
 	if err != nil {