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ARROW-1264: [Python] Raise exception in Python instead of aborting if cannot connect to Plasma store

cc @pcmoritz @robertnishihara

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #912 from wesm/ARROW-1264 and squashes the following commits:

bd134d7e [Wes McKinney] Add flags to disable certain classes of unit tests
1d9de777 [Wes McKinney] Raise exception in Python instead of aborting if cannot connect to Plasma store