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/repo/wiki.go

@@ -30,6 +30,10 @@ func (err ErrWikiAlreadyExist) Error() string {
 	return fmt.Sprintf("wiki page already exists [title: %s]", err.Title)
 }
 
+func (err ErrWikiAlreadyExist) Unwrap() error {
+	return util.ErrAlreadyExist
+}
+
 // ErrWikiReservedName represents a reserved name error.
 type ErrWikiReservedName struct {
 	Title string
@@ -45,6 +49,10 @@ func (err ErrWikiReservedName) Error() string {
 	return fmt.Sprintf("wiki title is reserved: %s", err.Title)
 }
 
+func (err ErrWikiReservedName) Unwrap() error {
+	return util.ErrInvalidArgument
+}
+
 // ErrWikiInvalidFileName represents an invalid wiki file name.
 type ErrWikiInvalidFileName struct {
 	FileName string
@@ -60,6 +68,10 @@ func (err ErrWikiInvalidFileName) Error() string {
 	return fmt.Sprintf("Invalid wiki filename: %s", err.FileName)
 }
 
+func (err ErrWikiInvalidFileName) Unwrap() error {
+	return util.ErrInvalidArgument
+}
+
 // WikiCloneLink returns clone URLs of repository wiki.
 func (repo *Repository) WikiCloneLink() *CloneLink {
 	return repo.cloneLink(true)