code golf: simplify FlowLsp.main_loop
code golf: simplify FlowLsp.main_loop

Summary: `match%lwt` should have the same behavior as before, where `main_handle` and `main_handle_error` are not part of the try/catch.

Reviewed By: gabelevi

Differential Revision: D17865022

fbshipit-source-id: 76071ae45c03d7dcf45b90ca9583c2d57adea57b

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow