Add manual runner removal

Runners can now be manually removed using the CLI. Some restrictions apply:

  * A runner must be idle in github. Github will not allow us to remove a runner
that is running a workflow.
  * The runner status must be "running"

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

errors/errors.go

@@ -29,7 +29,8 @@ var (
 	// ErrBadRequest is returned is a malformed request is sent
 	ErrBadRequest = NewBadRequestError("invalid request")
 	// ErrTimeout is returned when a timeout occurs.
-	ErrTimeout = fmt.Errorf("timed out")
+	ErrTimeout       = fmt.Errorf("timed out")
+	ErrUnprocessable = fmt.Errorf("cannot process request")
 )
 
 type baseError struct {