stylecontext: Be more graceful when save() isn't matched with restore()

stylecontext: Be more graceful when save() isn't matched with restore()

Instead of asserting, just print a g_warning() and try to work around
the problem.
I hope that g_warning() isn't too spammy for people that are hit with
it.

Also clarify the docs that not restore()ing after a save() is a bad
idea.

https://bugzilla.gnome.org/show_bug.cgi?id=743101