Pass --warn-error to Odoc

[Julow]

Hi !

The --warn-error option has been added to Odoc to turn warnings into errors (not yet released).
This PR add support for it in Dune.

By default the flag is not passed to Odoc, this can be changed with this in dune-project:

(odoc_warn_error true)

This flag is never passed in release mode.

Enabling this option and having an Odoc version that doesn't support it is an error. (no attempt is made to catch this)

[nojb]

Why not handle this as we do for ocamlc_flags, ocamlopt_flags and now also menhir_flags ? That is, why not add odoc_flags to the environment as well?

[Julow]

Not all Odoc commands take the --warn-error flag (this is probably a mistake) and Odoc commands have very few common options (so we would need at least odoc_compile_flags and odoc_html_flags).
Not exposing Odoc flags to the user allows Dune to change if they should be passed by default in the future.

[ghost]

This feels like more like a field for (env ...) than for the dune-project file though. In particular, so that we can choose different settings based on the build profile. The default could follow the default for ocaml compilation: warnings are errors only in dev mode

[Julow]

I opened an other PR for using (env ..): #3029
This PR can be closed.