Raise more helpfull exception on s.plot, df.plot ... without numerical data

[lodagro]

In [23]: df1 = pandas.DataFrame(list('abcd'))

In [24]: df1.plot()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
...
AttributeError: 'numpy.ndarray' object has no attribute 'find'

[cpcloud]

Also, passing an invalid value of kind gives a very cryptic error message.

[jreback]

closed by #3572