Include repr() of the actor in actor task errors

[richardliaw]

Describe your feature request

In many cases, IP/hostname/pid are not very interpretable.

Often times, in a user program, actors can have nice interpretable labels (either pre-set or determined during runtime).

Ideally, we can allow the user to do the following:

Traceback (most recent call last):
...
ray.exceptions.RayTaskError: ray::...() (label="data_sharder_rank:1")
...
ray.exceptions.RayTaskError: ray::...() (label="trainer_rank:4")

and the exception can also programmatically capture this label, like:

exc.cause.cause.cause.{label,pid,ip}

[ericl]

What if we just included the repr(actor) in the message?

[richardliaw]

Ah, that's a good fix.