message: generator_lib.py: correctly handle lack of --jinja2-path.
  Python's list.index() method raises an exception when the item isn't
  present in the list. Fix this check that was instead expecting a
  negative index.
diff: @@ -80,11 +80,15 @@ class Generator:
  # NOTE: If this argument appears several times, this only uses the first
  # value, while argparse would typically keep the last one!
  kJinja2Path = '--jinja2-path'
  +try:
  jinja2_path_argv_index = sys.argv.index(kJinja2Path)
  -if jinja2_path_argv_index >= 0:
  # Add parent path for the import to succeed.
  path = os.path.join(sys.argv[jinja2_path_argv_index + 1], os.pardir)
  sys.path.insert(1, path)
  +except ValueError:
  + # --jinja2-path isn't passed, ignore the exception and just import Jinja2
  + # assuming it already is in the Python PATH.
  + pass
  import jinja2
  