Nested Lwt_main.run check: handle exceptions
Nested Lwt_main.run check: handle exceptions

Caught by @talex5; see

  https://github.com/ocaml/opam-repository/pull/15541#issuecomment-566087278

c-cube/lwt,ocsigen/lwt,ocsigen/lwt,c-cube/lwt,ocsigen/lwt,c-cube/lwt