Return better error when server returns no offsets

Return OffsetOutOfRange instead of IncompleteResponse. Apparently when the
offset is out of range for an offset request, it doesn't return an error, just a
blank array, so translate that appropriately.

Fixes #123

client.go

@@ -207,7 +207,7 @@ func (client *Client) GetOffset(topic string, partitionID int32, where OffsetTim
 		return -1, block.Err
 	}
 	if len(block.Offsets) != 1 {
-		return -1, IncompleteResponse
+		return -1, OffsetOutOfRange
 	}
 
 	return block.Offsets[0], nil