st/texture-cache: Add missing error reporting

st/texture-cache: Add missing error reporting

st_texture_cache_load_file_sync_to_cogl_texture() loads a file into a
texture, but if pixbuf_to_st_content_image() failed, it'd return that it
failed, but wouldn't set an error, which would cause a null pointer
dereference when attempting to log the error message in the caller. Fix
this by setting an error on all error paths.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/9130
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4150>