Re-execute multi-queries if some query fails

Summary:
If some query fails, all next queries are not accessible.

Also don't report correct queries not returning results as errors.

Test Plan:

print_r(array_map('gettype', $conn->executeRawQueries(array(
  "SELECT 1",
  "DO",
  "DO SLEEP(5)",
))));
// And other permutations.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5900