Index without 0 in xerr/yerr causes KeyError
When creating an errorbar plot, if the data object being used for the errors (`xerr` or `yerr`) doesn't have 0 in its index, it produces a `KeyError` from matplotlib code. I can produce this with the following code sample:

``` python
import pandas as pd, numpy as np
i = np.array([1,2,3])
a = pd.DataFrame(i, index=i)
a.plot(yerr=a)
```

The underlying bug (if it is a bug) in matplotlib is responsible for several other issues, including #4493 and #6127, but this case is different because it uses only Pandas API methods, rather than passing a Pandas object to a matplotlib method. So it's a little harder to justify passing this off on matplotlib to fix, as was done in e.g. #6127.

If the "proper" fix ever is implemented in matplotlib code, it should solve this as well as #4493 and #6127 and all the others of that nature, but until that point, Pandas can work around it by converting the error object (if it is a `Series` or `DataFrame`) to an `ndarray`. I'm working on this in 4b04f80c41684ba9ab05ce8c87b17b96fde87290 but I'm not sure if there's a better way to fix it, or if this breaks something. (If so, the tests don't indicate it.)
