VAT ID validation fixes

Fix VAT ID being validated when it shouldn't be.
https://github.com/opencart/opencart/issues/943

Also fixes a problem where a ID of zero would pass validation.
Fix vat_validation function

Return 'invalid' on all errors to stop invalid characters, such as an
underscore, causing validation to pass. Fix problem where numbers with
spaces in return 'invalid'. Add missing ISO codes for Monaco and the
Isle of Man.