Propagate shader compile/link errors

And silence the no-error case.
diff --git a/compositor-drm.c b/compositor-drm.c
index 608ada9..c45b1d8 100644
--- a/compositor-drm.c
+++ b/compositor-drm.c
@@ -662,7 +662,8 @@
 	}
 	
 	/* Can't init base class until we have a current egl context */
-	wlsc_compositor_init(&ec->base, display);
+	if (wlsc_compositor_init(&ec->base, display) < 0)
+		return NULL;
 
 	if (create_outputs(ec) < 0) {
 		fprintf(stderr, "failed to create output for %s\n", path);