make returned errors public constants

[boindil]

declaring the returned errors as public constants would allow to add custom error handling while using this lib.

I for instance would like to handle the errors on .Get, but I'd like to decide what to do based on the specific error.

P.S.: is this project still alive? In #2 you said, that you were going to grow out the lib soon

P.P.S.: it has already been done for the DoesNotExist error, it's just not consistent.