If there's an error in cart, should not proceed to checkout

Hello. Where can I find the code that prohibits checkout if there's still an error in cart?

For example in minimum quantity. If you do not meet the requirements, it doesn't permit to checkout.

I've already started mine. The errors show correctly but permits checkout. I followed how minimum quantity works but mine still routes to checkout. Hope you can help me.

possibly with this, you can solve your problems.
http://forum.opencart.com/viewtopic.php?f=183&t=158485

Ernie
openshop.li