Verify the order before checking for error flags

[dd32]

While looking at the checkout process as part of #1296 I noticed that verify_order() is run after the check for error_flags, which means if verify_order() flags any issues with the order, the checkout still proceeds.

Some of the checks within the verify_order() method is duplicated within the checkout process already, but this ensures that any issues encountered during validation do properly stop the checkout process.

This might reduce the number of ticket_excess issues that some WordCamps can run into during high ticket loads.

[dd32]

This is less problematic than I originally thought.

verify_order() is being called multiple times.

During checkout; verify_order() is called:
https://github.com/WordPress/wordcamp.org/blob/production/public_html/wp-content/plugins/camptix/camptix.php#L5296-L5299

Errors from that instance of verify_order() are respected.

Then form_checkout() is run
https://github.com/WordPress/wordcamp.org/blob/production/public_html/wp-content/plugins/camptix/camptix.php#L5386-L5387

which then calls verify_order again:
https://github.com/WordPress/wordcamp.org/blob/production/public_html/wp-content/plugins/camptix/camptix.php#L7175-L7180

In that 2nd instance of verify_order running, it'll not respect any new errors thrown by verify_order.. but there shouldn't be any at that point, so the errors are being ignored, because there's unlikely to be any errors at all.

As a result, this is fairly safe to merge I think, as nothing should be changing, since the check is already happening.. but this is also now a low-priority thing