ERR: better error message when using invalid indexer in iloc
Not the most important isssue, but I find the error message when doing label based indexing with strings in `iloc` not very clear:

```
In [12]: s = pd.Series(range(5), index=pd.date_range("2012-01-01", periods=5))

In [15]: s.loc["2012-01-02":"2012-01-04"]
Out[15]: 
2012-01-02    1
2012-01-03    2
2012-01-04    3
Freq: D, dtype: int64

In [16]: s.iloc["2012-01-02":"2012-01-04"]
...
TypeError: cannot do slice indexing on <class 'pandas.tseries.index.DatetimeIndex'> with
these indexers [2012-01-02] of <class 'str'>
```

So I by accident used iloc instead of loc, but the error message did not directly make my error clear to me. I think it should just say something about that strings are not allowed in iloc. Because I *can* do slice indexing with strings on a DatetimeIndex (as the error message says I cannot), only not with iloc :-)

(on pandas 0.19.2)