ERR: better error reporting for passing list to level grouper on non-MultiIndex
`groupy(level=[0])` doesn't work on a unique index. I guess it is semantics whether one allows not only level=0, but also level=[0], but if not, the error message should be different:

```
df = pd.DataFrame(np.arange(0,9).reshape(3,3))
df.groupby(level=[0])
ValueError: level > 0 only valid with MultiIndex
```
