multiget does not always return RiakObjects, as stated in documentation

In a live application that we have deployed with Riak, we have noticed the following error message in the logs periodically, coming after some code that is expecting a RiakObject:
```
AttributeError: 'tuple' object has no attribute 'bucket'
```

We are attempting to access ```obj.bucket.name``` from an ```obj``` returned from  ```client.multiget```:

```
for obj in client.multiget(keys):
    if obj.bucket.name == test_bucket_name:
        ...
```

It is noted in the documentation that multiget should return a list of RiakObjects:
http://basho.github.io/riak-python-client/client.html#riak.client.RiakClient.multiget

However, it is clearly returning a ```tuple``` periodically according to the above examples.

I have been able to deterministically replicate this issue with a very short code sample, here:
```
from riak import RiakClient, RiakNode

client = RiakClient(nodes=[]) # Node list intentionally left empty
bkeys = [('testbucket', 'key%s' % i) for i in range(100)]

for obj in client.multiget(bkeys):
    print obj.bucket.name
```

Here is the result:
```
Traceback (most recent call last):
  File "test.py", line 7, in <module>
    print obj.bucket.name
AttributeError: 'tuple' object has no attribute 'bucket'
```

And here is what is contained in that tuple:
```
('testbucket', 'key73', ValueError('min() arg is an empty sequence',))
```

It seems that when encountering an exception, it returns a tuple which contains the exception in the third position.

The work-around for our code is obvious. We will simply check if the result is, in fact, a RiakObject before trying to access the bucket property (or maybe wrap in a try-except) and deal with whatever the error message is.

But, I am reporting this as a bug since the documentation suggests that multiget should only return RiakObjects and not tuples.

At the very least, the documentation should be updated.

I realize that there is not any straight-forward way to pass an exception regarding the failure of a single query in a parallel operation, but some documentation about this would be especially helpful for anyone else who is trying to use multiget properly.

Thanks!