Cleanup and organise common variables and errors

[snehmatic]

Some suggestions from @jakubdomanski. But there are definitely more that we can and should consolidate and organise.

1. Errors should be at package lvl = easier to compare in tests.
e.g.
var (
    ErrSubTaskNotFound = errors.New("subtask not found")
    ErrTaskNotFound    = errors.New("task not found")
)

2. Magic strings.
task.Status = "completed"
vs

const (
    StatusCompleted = "completed"
    StatusPending   = "pending"
)