PyDict should expose a wrapper over PyDict_GetItemWithError

[nagisa]

Currently PyDict::get_item wraps PyDict_GetItem and returns an Option::None when the key is missing or an exception occurs. It makes code incapable to differentiate between the two scenarios and code like this likely incorrect:

dict.get_item("key").ok_or_else(|| KeyError::py_err("key"))?;

[davidhewitt]

Marking for part of possible collections improvements in 0.15