compositor-wayland: clarify wl_display_connect() error

Clarify the error message to explicitly say one was trying to connect to
a parent Wayland compositor. This hopefully is a good enough hint on
what using the wayland-backend is trying to do.

Add the command line display option value and WAYLAND_DISPLAY values for
good measure. WAYLAND_SOCKET is not shown as libwayland-client removes
it.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Acked-by: Daniel Stone <daniels@collabora.com>

libweston/compositor-wayland.c

@@ -2541,7 +2541,10 @@ wayland_backend_create(struct weston_compositor *compositor,
 
 	b->parent.wl_display = wl_display_connect(new_config->display_name);
 	if (b->parent.wl_display == NULL) {
-		weston_log("failed to create display: %m\n");
+		weston_log("Error: Failed to connect to parent Wayland compositor: %m\n");
+		weston_log_continue(STAMP_SPACE "display option: %s, WAYLAND_DISPLAY=%s\n",
+				    new_config->display_name ?: "(none)",
+				    getenv("WAYLAND_DISPLAY") ?: "(not set)");
 		goto err_compositor;
 	}