Add scatter_contour error bars - #223

Follow-up on #222. I've added functionality for scatter_contour to handle error bars. This essentially amounts to changing the ax.scatter call to ax.errorbar and ensuring that user inputs (including None for xerr and/or yerr) are properly coerced and passed to the function call. I've tried to do this with minimal disruption to the organization of scatter_contour while replicating the direct function of ax.errorbar, but this definitely isn't the most parsimonious way to go about it.

Screen shots for each type of error bar input (as applied to SDSS Stripe 82 standard stars) are below. The only extra plot_arg that I've added for these figures is ecolor='gray'. Happy to add tests or further documentation!