Better error message when using DataFrame.hist() without numerical columns
pandas version: 0.16.2
matplotlib version 1.4.3 (and produced different error message on older version)

``` python
import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.rand(10,2))
df_o = df.astype(np.object)
df_o.hist()
```

```
ValueError                                Traceback (most recent call last)
<ipython-input-1-26253737011d> in <module>()
      4 df = pd.DataFrame(np.random.rand(10,2))
      5 df_o = df.astype(np.object)
----> 6 df_o.hist()

/usr/local/lib/python2.7/dist-packages/pandas/tools/plotting.pyc in hist_frame(data, column, by, grid, xlabelsize, xrot, ylabelsize, yrot, ax, sharex, sharey, figsize, layout, bins, **kwds)
   2764     fig, axes = _subplots(naxes=naxes, ax=ax, squeeze=False,
   2765                           sharex=sharex, sharey=sharey, figsize=figsize,
-> 2766                           layout=layout)
   2767     _axes = _flatten(axes)
   2768 

/usr/local/lib/python2.7/dist-packages/pandas/tools/plotting.pyc in _subplots(naxes, sharex, sharey, squeeze, subplot_kw, ax, layout, layout_type, **fig_kw)
   3244 
   3245     # Create first subplot separately, so we can share it if requested
-> 3246     ax0 = fig.add_subplot(nrows, ncols, 1, **subplot_kw)
   3247 
   3248     if sharex:

/usr/local/lib/python2.7/dist-packages/matplotlib/figure.pyc in add_subplot(self, *args, **kwargs)
    962                     self._axstack.remove(ax)
    963 
--> 964             a = subplot_class_factory(projection_class)(self, *args, **kwargs)
    965 
    966         self._axstack.add(key, a)

/usr/local/lib/python2.7/dist-packages/matplotlib/axes/_subplots.pyc in __init__(self, fig, *args, **kwargs)
     62                     raise ValueError(
     63                         "num must be 0 <= num <= {maxn}, not {num}".format(
---> 64                             maxn=rows*cols, num=num))
     65                 if num == 0:
     66                     warnings.warn("The use of 0 (which ends up being the "

ValueError: num must be 0 <= num <= 0, not 1
```
