Log messages will include the problematic CSS, simplifying debugging.
Especially helpful since some of our CSS is generated at runtime so it
could be difficult to examine the CSS "source".
```
info(gtk_ghostty_application): loading gtk-custom-css path=/home/ghostty/dev/ghostty/x.css
warning(gtk_ghostty_application): css parsing failed at <data>:2:3-14: gtk-css-parser-error-quark 4 No property named "border-poop"
* {
border-poop: 0;
warning(gtk_ghostty_application): css parsing failed at <data>:1:3-3:1: gtk-css-parser-warning-quark 1 Unterminated block at end of document
* {
border-poop: 0;
```
vs:
```
info(gtk_ghostty_application): loading gtk-custom-css path=/home/ghostty/dev/ghostty/x.css
warning(glib): WARNING: Gtk: Theme parser error: <data>:2:3-14: No property named "border-poop"
warning(glib): WARNING: Gtk: Theme parser warning: <data>:1:3-3:1: Unterminated block at end of document
```
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