Gave error handling a better place. From bootstrap.php to app.php
Spring cleaning in Storage.php

- Added a isValidColumn() function
- Added a queryParamOrder() parameter
- Made updateSingleValue more consistent with other functions
- updateSingleValue allows only valid column names.
- Refactored changeContent(), all calls now use updateSingleValue()
- Fixes #269, en passant.
- Tidied up some '$allowedcolumns' parts.