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/auth/source.go

@@ -12,6 +12,7 @@ import (
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/timeutil"
+	"code.gitea.io/gitea/modules/util"
 
 	"xorm.io/xorm"
 	"xorm.io/xorm/convert"
@@ -366,6 +367,11 @@ func (err ErrSourceNotExist) Error() string {
 	return fmt.Sprintf("login source does not exist [id: %d]", err.ID)
 }
 
+// Unwrap unwraps this as a ErrNotExist err
+func (err ErrSourceNotExist) Unwrap() error {
+	return util.ErrNotExist
+}
+
 // ErrSourceAlreadyExist represents a "SourceAlreadyExist" kind of error.
 type ErrSourceAlreadyExist struct {
 	Name string
@@ -381,6 +387,11 @@ func (err ErrSourceAlreadyExist) Error() string {
 	return fmt.Sprintf("login source already exists [name: %s]", err.Name)
 }
 
+// Unwrap unwraps this as a ErrExist err
+func (err ErrSourceAlreadyExist) Unwrap() error {
+	return util.ErrAlreadyExist
+}
+
 // ErrSourceInUse represents a "SourceInUse" kind of error.
 type ErrSourceInUse struct {
 	ID int64