Incorporated mongo exceptions

@jmikola Hi Jeremy. I'm helping the C++ team incorporating the mongo exceptions into the MongoCursor and MongoClient classes. I just modeled after your code in bson_decode.cpp but for some reason it's failing the test.sh test with this error:

<code>
Uncaught exception: Could not open extension ././mongo.so: ././mongo.so: undefined symbol: _ZN4HPHP20MongoCursorException3clsE
</code>