Add a ForwardTranslator specific key in the out OSW

[jmarrec]

Enhancement Request

Forward translator errors have no dedicated place in the out OSW, which makes it hard to diagnostic failures (or warnings) in that domain.

Detailed Description

It is possible that (in some cases?) these warnings are captured nowhere. Should probably test up the current situation before thinking about implementation.

Possible Implementation

Store the ft.warnings() / ft.errors in a dedicated OSW key. Requires changing the OSW schema.