Display error messages ingame

[sherpal]

When in game, many "errors" can come from the user action. Typically a user who "spam" an ability key, or tries to hit/heal some entity too far away, or not in sight.

When any of these errors occur in game, there should be, at the top of the screen, a toast-ish red message disappearing after some time. The display of these messages could perhaps depend on the kind of error.

This issue will require a little bit of rewriting in the code. Especially because currently there is no actual feedback onn why a given action can't be performed.