Add pre-migration step checking for null entity labels

[taylor-steve]

During migration testing on real world data, we are seeing:

Integrity constraint violation: 1048 Column 'title' cannot be null

• I think it would be a good idea to check for this directly
• It might be an even better idea to create a report that loads every entity we are migrating and validate them. If that covers title, then it is strictly better.

I think in D7 we might need to use Entity Metadata Wrappers to do this? Something like:

$entity = node_load($nid);
$wrapper = entity_metadata_wrapper('node', $entity);
$wrapper->validate();

Then we need to catch any exceptions thrown by validate? I haven't done this in D7 for too long, I don't remember exactly how it works.