Checks if the `wdiff` command can be found.

  Raises:

    WdiffNotFoundError: if ``wdiff`` is not found.

def check_for_wdiff():
  """
  Checks if the `wdiff` command can be found.

  Raises:

    WdiffNotFoundError: if ``wdiff`` is not found.

  """
  cmd = ['which', CMD_WDIFF]
  DEVNULL = open(os.devnull, 'wb')
  proc = sub.Popen(cmd, stdout=DEVNULL)
  proc.wait()
  DEVNULL.close()
  if proc.returncode != 0:
    msg = "the `{}` command can't be found".format(CMD_WDIFF)
    raise WdiffNotFoundError(msg)