2.1: 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 `EditCategory` method like `/vanilla/settings/editcategory/AnInvalidCategoryID`, a `fatal error in Gdn_Database.Query(); Unknown column 'the non-numeric argument' in 'on clause'` is shown.

These changes will show a friendly error message similar to how the friendly error message is displayed for non-numeric category IDs passed into the `DeleteCategory` method.