message: Avoid error if plugins .so module is not available
  This PR raises a warning instead of an error in case
  plugins .so module is not available, so that tensorflow-io
  package can be at least partially used with python-only
  functions.
diff: @@ -18,6 +18,7 @@ import os
  import ctypes
  import sys
  import inspect
  +import warnings
  import types
  import tensorflow as tf
  @@ -95,5 +96,8 @@ try:
  plugin_ops = _load_library("libtensorflow_io_plugins.so", "fs")
  except NotImplementedError as e:
  # Note: load libtensorflow_io.so imperatively in case of statically linking
  + try:
  core_ops = _load_library("libtensorflow_io.so")
  plugin_ops = _load_library("libtensorflow_io.so", "fs")
  + except NotImplementedError as e:
  + warnings.warn("file system plugins are not loaded: {}".format(e))
  