Improve handling of encoding errors with translation imports

[jeremydw]

WARNING: Start of line didn't match any expected keyword.
Error merging catalogs for locale -> ja
Traceback (most recent call last):
  File "grow", line 76, in <module>
  File "site-packages/click/core.py", line 764, in __call__
  File "site-packages/click/core.py", line 717, in main
  File "site-packages/click/core.py", line 1137, in invoke
  File "site-packages/click/core.py", line 1137, in invoke
  File "site-packages/click/core.py", line 956, in invoke
  File "site-packages/click/core.py", line 555, in invoke
  File "grow/commands/subcommands/translations_import.py", line 40, in translations_import
  File "grow/translations/catalog_holder.py", line 178, in import_translations
  File "grow/translations/importers.py", line 64, in import_path
  File "grow/translations/importers.py", line 148, in import_file
  File "grow/translations/importers.py", line 182, in import_content
  File "site-packages/babel/messages/pofile.py", line 377, in read_po
  File "site-packages/babel/messages/pofile.py", line 310, in parse
  File "site-packages/babel/messages/pofile.py", line 210, in _process_message_line
  File "site-packages/babel/messages/pofile.py", line 222, in _process_keyword_line
  File "site-packages/babel/messages/pofile.py", line 325, in _invalid_pofile
UnicodeEncodeError: 'ascii' codec can't encode character u'\ufeff' in position 28: ordinal not in range(128)
[76889] Failed to execute script grow