Upgrade should detect and throw error early on recommending schema correction if already broken

Fo example if 'show create table civicrm_event' output contains

KEY `FK_civicrm_event_payment_processor_id` ..

and not

CONSTRAINT `FK_civicrm_event_payment_processor_id` ..

is an indication that schema needs to rebuilding & we should point to http://wiki.civicrm.org/confluence/display/CRMDOC42/Ensuring+Schema+Integrity+on+Upgrades and not allow upgrade to proceed.

Above is just an example. We should do some checks with civicrm_contact table.

CRM_Core_DAO::checkConstraintExists() is an example of how this could be done.

Few related forum posts:
http://forum.civicrm.org/index.php/topic,25790.0.html
http://forum.civicrm.org/index.php/topic,25725.msg109040.html#msg109040