Display an alert when attempting to run a project with no main scene
Display an alert when attempting to run a project with no main scene

This gives visual feedback when not starting Godot from a terminal.
This could lead to confusion when placing a Godot binary within a
project folder that has no main scene defined.

(cherry picked from commit 87fb9297e3dcc28ddc844ecb9a3d4f9161f02da2)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot