Improve ERR message when DB entry already exists

[lebogg]

When creating a new DB entry that already exists we get the following error:

clouditor/persistence/gorm/gorm.go

Lines 187 to 193 in aa23384

	func (s *storage) Create(r any) (err error) {
	err = s.db.Create(r).Error
	if err != nil && (strings.Contains(err.Error(), "constraint failed: UNIQUE constraint failed") ||
	strings.Contains(err.Error(), "duplicate key value violates unique constraint")) {
	return persistence.ErrUniqueConstraintFailed
	}

clouditor/persistence/persistence.go

Line 35 in aa23384

ErrUniqueConstraintFailed = errors.New("unique constraint failed")

Couldn't we return sth. like "DB entry already exists" resulting in a more readable error message? Or are there other cases when this message would be inaccurate?
We could also forward this behavior to the individual endpoints which call this method, of course.