Warnings are not reported unless there are also errors

[paulyoung]

As mentioned in #439 (comment), this reports no warnings:

let foo = [1, true];
let bar = if true 1 else "2";

but this does (and an error)

let foo = [1, true];
let bar = if true 1 else "2";
let baz = qux;

I think the fix involves also including Diag.messages in the Ok case of compile_result.

https://github.com/dfinity-lab/actorscript/blob/191e237b7028ecb8defb80153a358f4b6554cf39/src/pipeline.ml#L451

[nomeata]

I might have messed this up in the latest refactoring of the pipeline. Will clean up once back from my vacation.