Make nonscalar field saves raise a more useful exception from LiskDAO

Summary:
If you do something like this:

// Missing $user->getPHID()!
$object->setUserPHID($user)->save();

...you get a very unhelpful exception:

Expected a scalar or null for %s conversion. Query: %s

This doesn't give you any hints about what's wrong. Instead, provide a more useful exception:

Unable to insert or update object of class DifferentialRevision, field 'title' has a nonscalar value.

Test Plan: {F87614}

Reviewers: hach-que, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7725