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>

modules/git/error.go

@@ -8,6 +8,8 @@ import (
 	"fmt"
 	"strings"
 	"time"
+
+	"code.gitea.io/gitea/modules/util"
 )
 
 // ErrExecTimeout error when exec timed out
@@ -41,6 +43,10 @@ func (err ErrNotExist) Error() string {
 	return fmt.Sprintf("object does not exist [id: %s, rel_path: %s]", err.ID, err.RelPath)
 }
 
+func (err ErrNotExist) Unwrap() error {
+	return util.ErrNotExist
+}
+
 // ErrBadLink entry.FollowLink error
 type ErrBadLink struct {
 	Name    string
@@ -87,6 +93,10 @@ func (err ErrBranchNotExist) Error() string {
 	return fmt.Sprintf("branch does not exist [name: %s]", err.Name)
 }
 
+func (err ErrBranchNotExist) Unwrap() error {
+	return util.ErrNotExist
+}
+
 // ErrPushOutOfDate represents an error if merging fails due to unrelated histories
 type ErrPushOutOfDate struct {
 	StdOut string