[watchman] Read_payload_too_long -> Watchman_error
[watchman] Read_payload_too_long -> Watchman_error

Summary:
replaces `Read_payload_too_long` with a `Watchman_error`, so that callers don't have to think about catching so many exceptions.

it was only raised by `blocking_read`, whose only caller is `get_changes`'s `on_alive` callback to `call_on_instance`, which catches both `Read_payload_too_long` and `Watchman_error`

Reviewed By: nmote

Differential Revision: D27611354

fbshipit-source-id: 3be56b4278b071bd10cc535e629d46117a43778e

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