Show Friendly Error Message for Non-Numeric Category ID

In Vanilla 2.1b3, if a non-numeric category ID argument is passed in to the `/vanilla/settings/editcategory/` method, a `fatal error in Gdn_Database.Query();` with `Unknown column 'the non-numeric argument' in 'on clause'` is shown.
Show Friendly Error Message for Non-Numeric Category ID

See other commit for `/applications/vanilla/controllers/class.settingscontroller.php`.
Merge pull request #1835 from shadowdare/patch-10

2.1: Show Friendly Error Message for Non-Numeric Category ID