fix: Improve provider apply() errors

[mfridman]

This PR adds a new error ErrNotApplied, that is returned from *provider.Apply() when trying to rollback a migration that has not been previously applied.

This makes it consistent with the up direction, which also returns an error (ErrAlreadyApplied) when trying to apply a migration that has previously been applied.

This set of comments aims to capture this behaviour:

//  1. direction is up
//    a. migration is applied, this is an error (ErrAlreadyApplied)
//    b. migration is not applied, apply it
//  2. direction is down
//    a. migration is applied, rollback
//    b. migration is not applied, this is an error (ErrNotApplied)