Returns the path to small thumbnail preview.

def thumbnail_preview(src_path):
  ''' Returns the path to small thumbnail preview. '''
  try:
    assert(exists(src_path))

    width = '1980'

    dest_dir = mkdtemp(prefix='pyglass')
    cmd = [QLMANAGE, '-t', '-s', width, src_path, '-o', dest_dir]
    assert(check_call(cmd) == 0)

    src_filename = basename(src_path)
    dest_list = glob(join(dest_dir, '%s.png' % (src_filename)))
    assert(dest_list)

    dest_path = dest_list[0]
    assert(exists(dest_path))

    return dest_path
  except:
    return None