Improve error message printing and suppression.

[robertnishihara]

Now if you do

@ray.remote
def f():
    raise Exception("ERROR")

The first time you call f.remote(), it will print

Remote function __main__.f failed with:

Traceback (most recent call last):
  File "<ipython-input-2-e4daaa126247>", line 3, in f
Exception: ERROR

The second time will print

Suppressing duplicate error message.

My concern with not printing anything at all the second time is that it may look like it succeeded (which could be confusing in an interactive interpreter environment.

Note that making the errors less verbose is especially important when using screen (where scrolling up is hard).

cc @joschu

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