ERR: more helpful error message on invalid access with a timestamp-like

[jreback]

related #7209

Might be a nicer error message (gist of this)

Hey you are trying to access a single element on AXIS 1, you can do it with a slice if you want, or use .loc to access the frame by row

In [3]: df = DataFrame(1,index=pd.date_range('20130101',periods=5),columns=['value'])

In [4]: df['20130102']
KeyError: '20130102'

In [5]: df['20130102':]
Out[5]: 
            value
2013-01-02      1
2013-01-03      1
2013-01-04      1
2013-01-05      1