Compat with brands - filters should be prepended with filter_

Conflicts:
	includes/widgets/class-wc-widget-price-filter.php
Fixes blank page error

Fix to avoid blank page error, apparently prevents a memory exausted condition.

Error source:
http://wordpress.org/support/topic/blank-page-after-return-form-cielo-buypage?replies=15

Some info:
http://stackoverflow.com/questions/8379829/domdocument-php-memory-leak

Another people using the same code: http://nathanstaines.com/articles/wordpress-word-count/

Cheers,
Gabriel
Checkout error hook

As committed on master branch: https://github.com/woothemes/woocommerce/commit/5f7753b343c4b2e0a7fc38acac39162bfa22be17