message: Improve error message when _load_library fails
diff: @@ -44,11 +44,13 @@ def _load_library(filename, lib="op"):
  else lambda f: tensorflow.compat.v1.load_file_system_library(f) == None
  # Try to load all paths for file, fail if none succeed
  + errs = []
  for f in filenames:
  try:
  l = load_fn(f)
  if l is not None:
  return l
  except errors.NotFoundError as e:
  - pass
  - raise NotImplementedError("unable to open file: ", filename)
  + errs.append(str(e))
  + raise NotImplementedError("unable to open file: " +
  + "{}, from paths: {}\ncaused by: {}".format(filename, filenames, errs))
  