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/util/error.go

@@ -0,0 +1,37 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package util
+
+import (
+	"errors"
+)
+
+// Common Errors forming the base of our error system
+//
+// Many Errors returned by Gitea can be tested against these errors
+// using errors.Is.
+var (
+	ErrInvalidArgument  = errors.New("invalid argument")
+	ErrPermissionDenied = errors.New("permission denied")
+	ErrAlreadyExist     = errors.New("resource already exists")
+	ErrNotExist         = errors.New("resource does not exist")
+)
+
+// SilentWrap provides a simple wrapper for a wrapped error where the wrapped error message plays no part in the error message
+// Especially useful for "untyped" errors created with "errors.New(…)" that can be classified as 'invalid argument', 'permission denied', 'exists already', or 'does not exist'
+type SilentWrap struct {
+	Message string
+	Err     error
+}
+
+// Error returns the message
+func (w SilentWrap) Error() string {
+	return w.Message
+}
+
+// Unwrap returns the underlying error
+func (w SilentWrap) Unwrap() error {
+	return w.Err
+}