[watchman] catch End_of_file
[watchman] catch End_of_file

Summary: if watchman closes our socket during `blocking_read`, we get an `End_of_file` exception. catch it and re-raise a `Watchman_error` instead, so that callers only have to worry about catching certain exceptions (and in the future, we can return a `result` instead).

Reviewed By: nmote

Differential Revision: D27610934

fbshipit-source-id: 647208d69bdefbf7f9b3d174c9d752d791367fd3

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