[tf.data] Move C++ code backing `tf.contrib.data.ignore_errors()` to contrib.

This change moves the `OpKernel` and `DatasetBase` implementations to
"tensorflow/contrib/data/kernels", where they are packaged as a custom
op library. This demonstrates (and enforces by continuous integration)
the ability to build a C++ Dataset implementation in a custom op
library.

Other contrib Dataset implementations will move in subsequent changes.

PiperOrigin-RevId: 184938885