Add exec id check on client

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

container.go

@@ -23,6 +23,7 @@ var (
 	ErrNoRunningTask     = errors.New("no running task")
 	ErrDeleteRunningTask = errors.New("cannot delete container with running task")
 	ErrProcessExited     = errors.New("process already exited")
+	ErrNoExecID          = errors.New("exec id must be provided")
 )
 
 type DeleteOpts func(context.Context, *Client, containers.Container) error

task.go

@@ -191,6 +191,9 @@ func (t *task) Delete(ctx context.Context) (uint32, error) {
 }
 
 func (t *task) Exec(ctx context.Context, id string, spec *specs.Process, ioCreate IOCreation) (Process, error) {
+	if id == "" {
+		return nil, ErrNoExecID
+	}
 	i, err := ioCreate()
 	if err != nil {
 		return nil, err