Inconsistent error raising in indexing

#### A small, complete example of the issue

``` python
x = np.random.randn(2, 2)
y = pd.Series(range(2))
y.loc[range(2)] = x # succeeds
y.loc[:] = x # fails
```
#### Expected Output

Since we're assigning a 2D array to a 1D vector that is smaller than the raveled array, I would expect both assignments to fail.
