Send task error instead of raw exception for signal

[ericl]

What do these changes do?

Exceptions are not always serializable, so we cannot send them safely. Send the RayTaskError instead.

Related issue number

Fixes #4149

[AmplabJenkins]

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[istoica]

This looks good to me.

[pcmoritz]

This is not currently passing tests, see https://travis-ci.com/ray-project/ray/jobs/180115893

[ericl]

The tests seem to only fail in python 2.

[robertnishihara]

I thought it was a cloudpickle bug, but it turns out to be a pickle bug.

In Python 2, this fails

import pickle

class A(Exception):
    def __init__(self, x):
        pass

a = A(1)

serialized = pickle.dumps(a)
pickle.loads(serialized)

It raises

Traceback (most recent call last):
  File "bug.py", line 10, in <module>
    pickle.loads(serialized)
  File "/home/ubuntu/miniconda/lib/python2.7/pickle.py", line 1388, in loads
    return Unpickler(file).load()
  File "/home/ubuntu/miniconda/lib/python2.7/pickle.py", line 864, in load
    dispatch[key](self)
  File "/home/ubuntu/miniconda/lib/python2.7/pickle.py", line 1139, in load_reduce
    value = func(*args)
TypeError: __init__() takes exactly 2 arguments (1 given)

https://stackoverflow.com/questions/41808912/cannot-unpickle-exception-subclass

[ericl]

Oh weird. I guess we could send function_name, backtrace as a tuple and construct RayTaskError on the other side then?

[robertnishihara]

I made it a string instead. What do you think about that?

[ericl]

Sounds good to me. We can iterate on this later.

[AmplabJenkins]

Test FAILed.
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Test FAILed.