Add an option to ignore TimeoutError exception

PyMongo always raises `TimeoutError` when you get wtimeout error from server due to `w=...` and `wtimeout=...` options in write operations. As for me it's almost always not the thing I'd want to get. I need an ability to ignore `TimeoutError` exception to inspect a getLastError object returned by the server like this:

```python
try:
    result = collection("foo").update(
        { "_id": "bar", "locked": { "$ne": True } },
        { "$set": { "locked": True } }, w="majority", wtimeout=...)

    if result["updatedExisting"]:
        log("The document is locked.")

        if result.get("wtimeout"):
            log("The changes are successfully written to primary, but not to majority.")
        else:
            log("The changes are successfully written to majority.")
except Exception:
    log("Failed to lock the document.")

try:
    result = collection("foo").insert(..., w="majority" wtimeout=..., full_response=True)

    if result.get("wtimeout"):
        log("The changes are successfully written to primary, but not to majority.")
    else:
        log("The changes are successfully written to majority.")
except Exception:
    log("Failed to insert the document.")
```

This pull request adds an ability to ignore `TimeoutError` and obtain a getLastError object like in the above example. If you are willing to accept it, I'll add tests to it.