Contribute pages do not check for required payment fields until transaction request

When completing a contribution page form, only non-payment fields are checked for whether they are required on the main form. Payment related fields (for direct payment systems) are not checked for whether they're required until the transaction data is submitted to the payment process, which subsequently returns some less attractive required field messages. It would be better for that required field check to happen on the main form by CiviCRM, rather than by the payment processor on subsequent pages.