Add error handling to backend creation

This commit adds two error-handling cases to the function
attempt_dmr_backend. Specifically:

- In the case where the number of found GPUs is zero, we now
  print a log message indicating this and return a NULL pointer
- In the case where we could not successfully create a backend
  on any GPU, we now log a message indicating this and return
  a NULL pointer

This allows us to provide more descriptive error messages,
as well as avoid a SEGFAULT (the function
`ensure_primary_backend_renderer_and_allocator` dereferences the pointer
given, which could be NULL until this patch) when these cases arise.

backend/backend.c

@@ -293,7 +293,12 @@ static struct wlr_backend *attempt_drm_backend(struct wl_display *display,
 		return NULL;
 	}
 
+	if (num_gpus == 0) {
+		wlr_log(WLR_ERROR, "Found 0 GPUs, cannot create backend");
+		return NULL;
+	} else {
 		wlr_log(WLR_INFO, "Found %zu GPUs", num_gpus);
+	}
 
 	struct wlr_backend *primary_drm = NULL;
 	for (size_t i = 0; i < (size_t)num_gpus; ++i) {
@@ -310,6 +315,10 @@ static struct wlr_backend *attempt_drm_backend(struct wl_display *display,
 
 		wlr_multi_backend_add(backend, drm);
 	}
+	if (!primary_drm) {
+		wlr_log(WLR_ERROR, "Could not successfully create backend on any GPU");
+		return NULL;
+	}
 
 	return ensure_backend_renderer_and_allocator(primary_drm);
 }