Warn, when using unknown key

[schmunk42]

Would help detecting errors, when migrating to the new config syntax, i.e I had the key for installer-paths wrong.

"fxp-asset": {
            "asset-installer-paths": {

[francoispluchino]

That is to say ? Can you develop?

[schmunk42]

Not in the very near future. But I'd really like to get 289 done.