Make every not exist error unwrappable to a fs.ErrNotExist (#20891)

A lot of our code is repeatedly testing if individual errors are
specific types of Not Exist errors. This is repetitative and unnecesary.
`Unwrap() error` provides a common way of labelling an error as a
NotExist error and we can/should use this.

This PR has chosen to use the common `io/fs` errors e.g.
`fs.ErrNotExist` for our errors. This is in some ways not completely
correct as these are not filesystem errors but it seems like a
reasonable thing to do and would allow us to simplify a lot of our code
to `errors.Is(err, fs.ErrNotExist)` instead of
`package.IsErr...NotExist(err)`

I am open to suggestions to use a different base error - perhaps
`models/db.ErrNotExist` if that would be felt to be better.


Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: delvh <dev.lh@web.de>

models/pull/automerge.go

@@ -90,7 +90,7 @@ func DeleteScheduledAutoMerge(ctx context.Context, pullID int64) error {
 	if err != nil {
 		return err
 	} else if !exist {
-		return db.ErrNotExist{ID: pullID}
+		return db.ErrNotExist{Resource: "auto_merge", ID: pullID}
 	}
 
 	_, err = db.GetEngine(ctx).ID(scheduledPRM.ID).Delete(&AutoMerge{})