Fix unknown target platform message

The message displayed for an unknown target platform is currently:
The target platform 'cap-5.9.3' is unknown, please fix it on the Project Settings page.

It would be nicer with something like:
The target platform 'cap-5.9.3' is unknown, please select another target version in your Project Settings.

Also when target platform is a recent fast-track, another message should be displayed (currently broken because hard-coded on 5.7.x target platforms).