BUG: scatter with errorbar raises IndexError
Closes #8081.

Also fixes a color problem with `c` kw found in #7780. (But should work with single color. It looks `errorbar` doesn't accept multiple colors).

Will review later today.

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Only slightly related, but what are your thoughts on on `Series.plot(kind='scatter')`? Right now it raises a `ValueError`, but I expected it to work with `x=series.index, y=series.values`.
