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/webauthn.go

@@ -11,6 +11,7 @@ import (
 
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/modules/timeutil"
+	"code.gitea.io/gitea/modules/util"
 
 	"github.com/duo-labs/webauthn/webauthn"
 	"xorm.io/xorm"
@@ -29,6 +30,11 @@ func (err ErrWebAuthnCredentialNotExist) Error() string {
 	return fmt.Sprintf("WebAuthn credential does not exist [credential_id: %x]", err.CredentialID)
 }
 
+// Unwrap unwraps this as a ErrNotExist err
+func (err ErrWebAuthnCredentialNotExist) Unwrap() error {
+	return util.ErrNotExist
+}
+
 // IsErrWebAuthnCredentialNotExist checks if an error is a ErrWebAuthnCredentialNotExist.
 func IsErrWebAuthnCredentialNotExist(err error) bool {
 	_, ok := err.(ErrWebAuthnCredentialNotExist)