Warn on resource deadlock; improve object store error messages (#5555)

* wip

* wip

* wip

* wip

* wip

* add impl

* second

* warn once

python/ray/exceptions.py

@@ -105,8 +105,14 @@ class UnreconstructableError(RayError):
         self.object_id = object_id
 
     def __str__(self):
-        return ("Object {} is lost (either evicted or explicitly deleted) and "
-                + "cannot be reconstructed.").format(self.object_id.hex())
+        return (
+            "Object {} is lost (either LRU evicted or deleted by user) and "
+            "cannot be reconstructed. Try increasing the object store "
+            "memory available with ray.init(object_store_memory=<bytes>) "
+            "or setting object store limits with "
+            "ray.remote(object_store_memory=<bytes>). See also: {}".format(
+                self.object_id.hex(),
+                "https://ray.readthedocs.io/en/latest/memory-management.html"))
 
 
 RAY_EXCEPTION_TYPES = [