SPARK-1115: Catch depickling errors

This surroungs the complete worker code in a try/except block so we catch any error that arrives. An example would be the depickling failing for some reason

@JoshRosen

Author: Bouke van der Bijl <boukevanderbijl@gmail.com>

Closes #644 from bouk/catch-depickling-errors and squashes the following commits:

f0f67cc [Bouke van der Bijl] Lol indentation
0e4d504 [Bouke van der Bijl] Surround the complete python worker with the try block
(cherry picked from commit 12738c1aec136acd7f2e3e2f8f2b541db0890630)

Signed-off-by: Josh Rosen <joshrosen@apache.org>