[xray] Add error table and push error messages to driver through node manager. (#2256)

* Fix documentation indentation.

* Add error table to GCS and push error messages through node manager.

* Add type to error data.

* Linting

* Fix failure_test bug.

* Linting.

* Enable one more test.

* Attempt to fix doc building.

* Restructuring

* Fixes

* More fixes.

* Move current_time_ms function into util.h.

python/ray/ray_constants.py

@@ -5,6 +5,8 @@ from __future__ import print_function
 
 import os
 
+import ray
+
 
 def env_integer(key, default):
     if key in os.environ:
@@ -12,6 +14,9 @@ def env_integer(key, default):
     return default
 
 
+ID_SIZE = 20
+NIL_JOB_ID = ray.ObjectID(ID_SIZE * b"\x00")
+
 # If a remote function or actor (or some other export) has serialized size
 # greater than this quantity, print an warning.
 PICKLE_OBJECT_WARNING_SIZE = 10**7