Feature Request: Expand option for __error__

[eric-downes]

Right now, to access an error of a pandas_mapper object m, I have to do something like this: m.errors.__error__.iloc[0] which returns a dict

Since the __error__ column is a dict that always has the same columns, it would be more convenient to have it expand those keys into dataframe columns (you can prepend "pdmap_err_" to each key first if you're afraid of clobbering existing columns.

At least, it would be nice to have such an option even if it weren't the default.