snapctl should return a better error if a task manifest is passed in without a schedule

[geauxvirtual]

Currently, if a task manifest does not include a schedule, snapctl will return runtime error: invalid memory address or nil pointer dereference. When creating a task, there is no validation that the task manifest passed in included a schedule. An error should be returned to the user saying the task manifest needs to include a schedule instead of a vague runtime error.