Fix debug mode msg import/export

[chrisfls]

PR #437 is introducing great improvements for context menu handling, but it's also breaking our msg import/export capabilities.

This functionality isn't being used yet, but it will be a great help for debugging user interface issues once we start a staging server.

And hey, it's not harming our codebase maintability like the error message claims to be, but I'm concerned about using an unsuported feature: it may break our game in the far, far future when elm starts shipping with wasm support.

My proposal is to fix that once we start a staging server.