TaskCancellation (#7669)

* Smol comment

* WIP, not passing ray.init

* Fixed small problem

* wip

* Pseudo interrupt things

* Basic prototype operational

* correct proc title

* Mostly done

* Cleanup

* cleaner raylet error

* Cleaning up a few loose ends

* Fixing Race Conds

* Prelim testing

* Fixing comments and adding second_check for kill

* Working_new_impl

* demo_ready

* Fixing my english

* Fixing a few problems

* Small problems

* Cleaning up

* Response to changes

* Fixing error passing

* Merged to master

* fixing lock

* Cleaning up print statements

* Format

* Fixing Unit test build failure

* mock_worker fix

* java_fix

* Canel

* Switching to Cancel

* Responding to Review

* FixFormatting

* Lease cancellation

* FInal comments?

* Moving exist check to CoreWorker

* Fix Actor Transport Test

* Fixing task manager test

* chaning clock repr

* Fix build

* fix white space

* lint fix

* Updating to medium size

* Fixing Java test compilation issue

* lengthen bad timeouts

python/ray/exceptions.py

@@ -16,6 +16,23 @@ class RayConnectionError(RayError):
     pass
 
 
+class RayCancellationError(RayError):
+    """Raised when this task is cancelled.
+
+    Attributes:
+        task_id (TaskID): The TaskID of the function that was directly
+            cancelled.
+    """
+
+    def __init__(self, task_id=None):
+        self.task_id = task_id
+
+    def __str__(self):
+        if self.task_id is None:
+            return "This task or its dependency was cancelled by"
+        return "Task: " + str(self.task_id) + " was cancelled"
+
+
 class RayTaskError(RayError):
     """Indicates that a task threw an exception during execution.