Provide information about wrong value passed to assert_instances_of()

Test Plan:

assert_instances_of(array(1), 'stdClass');
assert_instances_of(array(new stdClass), 'C');
assert_instances_of(array(new stdClass), 'stdClass');

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2554