include stack traces in watchman connection errors
include stack traces in watchman connection errors

Summary: i want to move these catches closer to where they're raised, and either return results or raise `watchman.ml`-specific errors. but i don't know where they're coming from, so i'm including the backtraces in the logs.

Reviewed By: nmote

Differential Revision: D27567684

fbshipit-source-id: d6dde0e65b3ce5103f2492bc475082c259c225aa

facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow