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/user/openid.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 
 	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/modules/util"
 )
 
 // ErrOpenIDNotExist openid is not known
@@ -65,6 +66,10 @@ func (err ErrOpenIDAlreadyUsed) Error() string {
 	return fmt.Sprintf("OpenID already in use [oid: %s]", err.OpenID)
 }
 
+func (err ErrOpenIDAlreadyUsed) Unwrap() error {
+	return util.ErrAlreadyExist
+}
+
 // AddUserOpenID adds an pre-verified/normalized OpenID URI to given user.
 // NOTE: make sure openid.URI is normalized already
 func AddUserOpenID(ctx context.Context, openid *UserOpenID) error {