nonsensical error message for invalid input to plt.bar

mpl2.0b3, py3.5.2

```
In [1]: plt.bar(range(3), 1)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<elided>
ValueError: incompatible sizes: argument 'height' must be length 3 or scalar
```

Well, it is a scalar...
