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/error.go

@@ -6,6 +6,8 @@ package user
 
 import (
 	"fmt"
+
+	"code.gitea.io/gitea/modules/util"
 )
 
 //  ____ ___
@@ -30,6 +32,11 @@ func (err ErrUserAlreadyExist) Error() string {
 	return fmt.Sprintf("user already exists [name: %s]", err.Name)
 }
 
+// Unwrap unwraps this error as a ErrExist error
+func (err ErrUserAlreadyExist) Unwrap() error {
+	return util.ErrAlreadyExist
+}
+
 // ErrUserNotExist represents a "UserNotExist" kind of error.
 type ErrUserNotExist struct {
 	UID   int64
@@ -47,6 +54,11 @@ func (err ErrUserNotExist) Error() string {
 	return fmt.Sprintf("user does not exist [uid: %d, name: %s, keyid: %d]", err.UID, err.Name, err.KeyID)
 }
 
+// Unwrap unwraps this error as a ErrNotExist error
+func (err ErrUserNotExist) Unwrap() error {
+	return util.ErrNotExist
+}
+
 // ErrUserProhibitLogin represents a "ErrUserProhibitLogin" kind of error.
 type ErrUserProhibitLogin struct {
 	UID  int64
@@ -63,6 +75,11 @@ func (err ErrUserProhibitLogin) Error() string {
 	return fmt.Sprintf("user is not allowed login [uid: %d, name: %s]", err.UID, err.Name)
 }
 
+// Unwrap unwraps this error as a ErrPermission error
+func (err ErrUserProhibitLogin) Unwrap() error {
+	return util.ErrPermissionDenied
+}
+
 // ErrUserInactive represents a "ErrUserInactive" kind of error.
 type ErrUserInactive struct {
 	UID  int64
@@ -78,3 +95,8 @@ func IsErrUserInactive(err error) bool {
 func (err ErrUserInactive) Error() string {
 	return fmt.Sprintf("user is inactive [uid: %d, name: %s]", err.UID, err.Name)
 }
+
+// Unwrap unwraps this error as a ErrPermission error
+func (err ErrUserInactive) Unwrap() error {
+	return util.ErrPermissionDenied
+}