Expose a storage.ErrNotExists for non existing documents

[bon-ami]

Is your feature request related to a problem? Please describe:

My code cannot check an error returned by, such as app.Storage().Save() with errors.Is()

Is it possible to construct a solution with the existing API?

Such as return os.ErrNotExist for document with name * does not exist

Describe the solution you'd like to see:

Please use standard errors from Go packages instead of using errors.New()

[andydotxyz]

I'm not sure that I would agree that a document not existing is an os.ErrNotExist error.
However it is true that we could use sentinel / defined error types better.

We could add a Exists to Docs I guess, but the list of available documents is returned by List.

[Jacalz]

We might want tot just create our own storage.ErrNotExists variable (or similar) and then just return that instead. That would solve the issue presented here.

[bon-ami]

We might want tot just create our own storage.ErrNotExists variable (or similar) and then just return that instead. That would solve the issue presented here.

sounds great

[andydotxyz]

Ready for next release on develop branch