Make db.ErrNotExist reusable

[wolfogre]

This is a proposal.

There are lots of ErrXXXNotExist in models, like

// ErrUserNotExist represents a "UserNotExist" kind of error.
type ErrUserNotExist struct {
	UID   int64
	Name  string
	KeyID int64
}

// IsErrUserNotExist checks if an error is a ErrUserNotExist.
func IsErrUserNotExist(err error) bool {
	_, ok := err.(ErrUserNotExist)
	return ok
}

But sometimes I don't care what kind of "NotExist" the error is, I just want to know if it is "NotExist".

If this PR had been merged, I could do that like this:

type ErrUserNotExist struct {
	db.ErrNotExist
	// ...
}

var err error = ErrUserNotExist{}
IsErrUserNotExist(err) // true
db.IsErrNotExist(err) // true
IsErrRepoNotExist(err) // false

[lafriks]

Maybe we could add interface that these kind of errors could implement?

[delvh]

See also #20891

[wolfogre]

See also #20891

Sorry I missed it, it looks better, I'll close this one.