GtkCssProvider: Improve error handling and reporting.
GtkCssProvider: Improve error handling and reporting.

Nicer error reports are provided, and a GError can be spread from
anywhere in parsing, so over time more precise and meaningful
messages can be produced.

jadahl/gtk,bratsche/gtk-,davidt/gtk,Lyude/gtk-,Sidnioulz/SandboxGtk,jigpu/gtk,davidgumberg/gtk,chergert/gtk,jessevdk/gtk,jessevdk/gtk,grubersjoe/adwaita,davidt/gtk,bratsche/gtk-,davidgumberg/gtk,davidgumberg/gtk,bratsche/gtk-,Sidnioulz/SandboxGtk,msteinert/gtk,alexlarsson/gtk,Adamovskiy/gtk,jigpu/gtk,jessevdk/gtk,jessevdk/gtk,alexlarsson/gtk,davidgumberg/gtk,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,chergert/gtk,Lyude/gtk-,Adamovskiy/gtk,ahodesuka/gtk,Lyude/gtk-,Sidnioulz/SandboxGtk,ahodesuka/gtk,grubersjoe/adwaita,Lyude/gtk-,jadahl/gtk,ebassi/gtk,davidgumberg/gtk,msteinert/gtk,grubersjoe/adwaita,ebassi/gtk,Distrotech/gtk2,alexlarsson/gtk,davidt/gtk,chergert/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,ahodesuka/gtk,ahodesuka/gtk,Distrotech/gtk2,davidgumberg/gtk,jadahl/gtk,grubersjoe/adwaita,jigpu/gtk,msteinert/gtk,jadahl/gtk,ebassi/gtk,Distrotech/gtk2,Sidnioulz/SandboxGtk,alexlarsson/gtk,Adamovskiy/gtk,bratsche/gtk-,ahodesuka/gtk,Sidnioulz/SandboxGtk,chergert/gtk,ebassi/gtk,Lyude/gtk-,simokivimaki/gtk,davidgumberg/gtk,Distrotech/gtk2,simokivimaki/gtk,jigpu/gtk,msteinert/gtk,davidgumberg/gtk,alexlarsson/gtk,simokivimaki/gtk,jadahl/gtk,davidt/gtk,Lyude/gtk-,grubersjoe/adwaita,simokivimaki/gtk,bratsche/gtk-,Adamovskiy/gtk,ahodesuka/gtk,Distrotech/gtk2,jigpu/gtk,chergert/gtk,jessevdk/gtk,msteinert/gtk,ebassi/gtk,jessevdk/gtk,jadahl/gtk,Distrotech/gtk2,jigpu/gtk,ebassi/gtk,bratsche/gtk-,Adamovskiy/gtk,davidt/gtk,Lyude/gtk-,ahodesuka/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidt/gtk,grubersjoe/adwaita,jigpu/gtk,chergert/gtk,Adamovskiy/gtk,msteinert/gtk,jadahl/gtk,chergert/gtk,jessevdk/gtk,Adamovskiy/gtk,ahodesuka/gtk,jigpu/gtk,jadahl/gtk,simokivimaki/gtk,Lyude/gtk-,simokivimaki/gtk