CLN/BUG/ENH: Raise AttributeError with pd.options
Fixes #5182, closes #5186.

It's called OptionError now and it subclasses both AttributeError (for
hasattr) and KeyError (for backwards compatibility). Hurray!

Anybody object to `OptionError(AttributeError, KeyError)`? Backwards _and_ forwards compatible -- whee!
 Exception abuse! but ok! practical beats purity!
 this is fine for #5182, but I don't think we should fix for ipython 0.2-dev when they are at least a year away and this interface could change

but maybe lodge an objection as an issue there? (or at least an incompat change they are doing)...?
 yeah, but generally it's surprising to have attribute access raise KeyErrors anyways.
 @jtratner yep...that's why agree for #5182 it makes sense
