Remove string checking in API error handling

Use strongly typed errors to set HTTP status codes.
Error interfaces are defined in the api/errors package and errors
returned from controllers are checked against these interfaces.

Errors can be wraeped in a pkg/errors.Causer, as long as somewhere in the
line of causes one of the interfaces is implemented. The special error
interfaces take precedence over Causer, meaning if both Causer and one
of the new error interfaces are implemented, the Causer is not
traversed.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/stop.go

@@ -2,12 +2,10 @@ package daemon
 
 import (
 	"context"
-	"fmt"
-	"net/http"
 	"time"
 
-	"github.com/docker/docker/api/errors"
 	containerpkg "github.com/docker/docker/container"
+	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 )
 
@@ -23,15 +21,14 @@ func (daemon *Daemon) ContainerStop(name string, seconds *int) error {
 		return err
 	}
 	if !container.IsRunning() {
-		err := fmt.Errorf("Container %s is already stopped", name)
-		return errors.NewErrorWithStatusCode(err, http.StatusNotModified)
+		return containerNotModifiedError{running: false}
 	}
 	if seconds == nil {
 		stopTimeout := container.StopTimeout()
 		seconds = &stopTimeout
 	}
 	if err := daemon.containerStop(container, *seconds); err != nil {
-		return fmt.Errorf("Cannot stop container %s: %v", name, err)
+		return errors.Wrapf(systemError{err}, "cannot stop container: %s", name)
 	}
 	return nil
 }