test_large_inserts_ordered fails with a 2.4 mongos. mongos responds to getLastError after an oversized insert:

	{ err: "assertion src/mongo/s/strategy_shard.cpp:461", n: 0, ok: 1.0 }

There's an "err" but no "code", so _mongoc_write_result_merge_legacy skips it. There may be more cases where the server returns an err with no code, audit all.