Instead of an error vfunc, have the tokenizer vfunc take a GError
argument. Note that even when an error is returned, there is still a
token to be read.
This library is meant to be the new CSS library that gets used from GDK,
GSK and GTK for string printing and parsing.
As a first step, move GtkCssProviderError into it.
While doing so, split it into GtkCssParserError (for critical problems)
and GtkCssParserWarning (for non-critical problems).
