eio_luv: reading a stream after closing it crashes the event loop

[talex5]

Seen in mirage/ocaml-cohttp#965:

Uncaught exception in run loop:
Exception: Invalid_argument("read_start: handle used after calling close!")
Raised at Stdlib.invalid_arg in file "stdlib.ml", line 30, characters 20-45
Called from Eio_luv.Low_level.Stream.read_into.(fun) in file "vendor/eio/lib_eio_luv/eio_luv.ml", line 512, characters 32-62

We should just discontinue the fiber with the exception, not kill the whole loop.

/cc @bikallem

[bikallem]

@talex5 do you mean here?

eio/lib_eio_luv/eio_luv.ml

Line 377 in a36443f

| { fd = `Closed ; _ } -> invalid_arg (op ^ ": file descriptor used after calling close!")

It seems to be used in a few other places, i.e. getting the handle. It it okay if we discontinue in other use cases too?

[talex5]

Yes, but it's not worth spending a lot of time improving eio_luv, since we're planning to replace it anyway.