Improve tfdbg's handling of runtime errors

* In some cases the RuntimeError object (tf_error in cli_shared.py) doesn't have
  the op or its name available. Handle that situation properly.
* Previously, we used client graphs in the debugger CLI whenever it's available. This
  has caused issues in which the device names
  (e.g., "/device:GPU:0" vs "/job:localhost/replica:0/task:0/device:CPU:0").
  This CL fixes that by favoring the runtime graph on the disk over the client graph.
  The former has the actual device names.
  Use the latter only if the former isn't available for some reason (e.g.,
  writing graph to the disk failed.)

PiperOrigin-RevId: 200128582