Show notification if no active Manim session when trying to quit

[Splines]

Fixes #71, i.e. we now show an information message whenever the user tries to quit the scene while no active scene is running.

I've already implemented such a notification beforehand but accidentally removed it in #48 when fixing the exit scene behavior where the exitScene() was rewritten.

Off-topic

The error message notification when clearing the scene while no active session exists, is changed to an information message. An information message is also used for the notification upon quitting (when no active session exists).

[VladimirFokow]

oh,
I liked the orange warning
(instead of the blue "i" icon which is now in both "clear" and "quit")

Can we make them back to the warning?
(But it's personal preference, so approving)

[Splines]

But you're right, it's more than just an information. I'd consider it a warning, so I changed it to a warning message now.