[tf.data] Improve error messages in iterator_ops.py.
[tf.data] Improve error messages in iterator_ops.py.

PiperOrigin-RevId: 397202544
Change-Id: I7ec1271bc571682102a748127517898304bb7b35

yongtang/tensorflow,yongtang/tensorflow,paolodedios/tensorflow,yongtang/tensorflow,tensorflow/tensorflow,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_saved_model,Intel-Corporation/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_saved_model,karllessard/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-pywrap_saved_model,yongtang/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-pywrap_tf_optimizer,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow,paolodedios/tensorflow,tensorflow/tensorflow-pywrap_saved_model,gautam1858/tensorflow,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow,yongtang/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,Intel-Corporation/tensorflow,tensorflow/tensorflow-pywrap_saved_model,yongtang/tensorflow,yongtang/tensorflow,karllessard/tensorflow,karllessard/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow,paolodedios/tensorflow,Intel-tensorflow/tensorflow,paolodedios/tensorflow,karllessard/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,yongtang/tensorflow,paolodedios/tensorflow,yongtang/tensorflow,tensorflow/tensorflow,karllessard/tensorflow,tensorflow/tensorflow-pywrap_saved_model,gautam1858/tensorflow,gautam1858/tensorflow,Intel-Corporation/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,gautam1858/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-pywrap_saved_model,Intel-Corporation/tensorflow,paolodedios/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,karllessard/tensorflow,Intel-tensorflow/tensorflow,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_saved_model,yongtang/tensorflow,Intel-tensorflow/tensorflow,Intel-Corporation/tensorflow,paolodedios/tensorflow,Intel-Corporation/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,paolodedios/tensorflow,gautam1858/tensorflow,tensorflow/tensorflow,Intel-Corporation/tensorflow,paolodedios/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,gautam1858/tensorflow,paolodedios/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,karllessard/tensorflow,gautam1858/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,karllessard/tensorflow,Intel-Corporation/tensorflow,yongtang/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-pywrap_saved_model,paolodedios/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,Intel-tensorflow/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow,karllessard/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-pywrap_saved_model,karllessard/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,karllessard/tensorflow,Intel-tensorflow/tensorflow