Fixes related to validating worksheet name uniqueness

0523e90 (Edward Anderson, 5 minutes ago)
   Worksheet name uniqueness doesn't apply to itself

   This constraint would prevent you from creating a new worksheet with the
   name that it would have gotten by default.

cac5b8a (Edward Anderson, 33 minutes ago)
   Don't add worksheets to the workbook when initialize fails

   Creating a worksheet with an invalid name would still add it to the
   workbook, despite raising an exception.