xw.Book() and xw.books.open() raise different error if file does not exist

[fzumstein]

should be aligned

xw.books.open("Test.xlsx") should open up this file if it isn't open. Right now just returning AttributeError.
xw.Book("Test.xlsx") opens up Test.xlsx as expected.

I cloned this to my Git and rand the unit tests. They work fine and now so does xw.books.open("Test.xlsx"). Seems something was causing my previous calls to xl.books.open() to attempt to reference a None object.

[fzumstein]

xw.books is the collection of the active app. If there's no Excel instance running, i.e. there's no app, it will not work. xw.Book is a convenience class that takes care of instantiating a new app if there isn't one yet.