Instead of just checking that the line of the error message is correct,
assert that start and end position are on the correct character offset.
Also fix all the tests to conform to this.
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).
