RPC builds don't transfer warnings to the client

Expected Behavior

If there is a non-fatal warning during build, the RPC server should probably relay those warnings to the client, and the client shouldn't just say "Success!"

Actual Behavior

-w +26 means the warning is activated but non-fatal
-w @26 would mean it's activated and fatal
  $ echo "(executable (flags (-w +26)) (name foo))" > dune

  $ echo "let () = let x = 4 in ()" > foo.ml

  $ start_dune

We should see the warning here
  $ dune build
  Success

Modifying foo.ml to invalidate caches
  $ echo "let () = let x = 6 in ()" > foo.ml

We should also see the warning here
  $ dune rpc build
  Success

  $ stop_dune
  File "foo.ml", line 1, characters 13-14:
  1 | let () = let x = 4 in ()
                   ^
  Warning 26 [unused-var]: unused variable x.
  Success, waiting for filesystem changes...
  Success, waiting for filesystem changes...

Found while trying to make a cram test that would only trigger #12578.

Start by running the cram reproducer with dune build and dune rpc build, focusing on how the RPC client reports build results and warnings. The change is done when a non-fatal Warning 26 from the shown foo.ml example is relayed to the client instead of reporting only Success.