Update download-spreadsheet.py

Removing the norm definition, since it's not used.

Added in some error checking, this specifically gets rid of the BOM that can cause "unicodeEncodeError 'ascii' codec can't encode character u'/ufeff' in position 0: ordinal not in range(128)"

This has been tested and runs well on Google spreadsheets with more than one worksheet (>1 tab), that had those errors.