Better error reporting for issue selecting a scanner

[vletoux]

If you try to run a scanner which has the wrong allowed asset type compared to the asset, you can save the scan definition.

There is no error triggered in the save process.

I was able to do it by editing an existing scan and changed the assets by a one having a wrong type.

By the way, when the scanner is then launched, an error is shown:

Despite the tip indicated in the error message is incorrect.

If the engine cannot be selected, I suggest to specify the reason in the error message.