Raise exception in Workbook.save() when saving a workbook with no worksheets

[mapellegrini]

I ran into this problem when learning how to use xlwt. Currently it throws an unhelpful index error:

import xlwt
wb=xlwt.Workbook()
wb.save("/tmp/test.xls")
Traceback (most recent call last):
File "", line 1, in
File "/usr/local/lib/python2.7/dist-packages/xlwt/Workbook.py", line 710, in save
doc.save(filename_or_stream, self.get_biff_data())
File "/usr/local/lib/python2.7/dist-packages/xlwt/Workbook.py", line 680, in get_biff_data
self.__worksheets[self.__active_sheet].selected = True
IndexError: list index out of range

[coveralls]

Coverage decreased (-0.003%) to 59.507% when pulling 179c87e on mapellegrini:master into 0fc028f on python-excel:master.

[mapellegrini]

Here is the relevant test (carried out on xlwt as cloned from my github repo to /tmp):

import sys
sys.path=["/tmp/xlwt"] + sys.path
import xlwt
wb=xlwt.Workbook()
wb.save("/tmp/test.xls")
Traceback (most recent call last):
File "", line 1, in
File "/tmp/xlwt/xlwt/Workbook.py", line 710, in save
raise Exception("Can't save a workbook that contains no worksheets")
Exception: Can't save a workbook that contains no worksheets

[jdufresne]

Thanks for the contribution. Can you add a test to tests/ please?