container wait: improve error message

Improve the error message when looking up the exit code of a container.
The state of the container may help us track down containers#14859 which flakes
rarely and is impossible to reproduce on my machine.

[NO NEW TESTS NEEDED]

Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>

libpod/container_api.go

@@ -555,7 +555,7 @@ func (c *Container) WaitForExit(ctx context.Context, pollInterval time.Duration)
 				// The container never ran.
 				return true, 0, nil
 			}
-			return true, -1, err
+			return true, -1, fmt.Errorf("%v (container in state %s)", err, c.state.State)
 		}
 
 		return true, exitCode, nil