Warn user if manual device selection failed

Author: stduhpf

tools/mtmd/clip.cpp

@@ -389,6 +389,9 @@ struct clip_ctx {
             auto backend_name = std::getenv("MTMD_BACKEND_DEVICE");
             if (backend_name != nullptr) {
                 backend = ggml_backend_init_by_name(backend_name, nullptr);
+                if (!backend) {
+                    LOG_WRN("%s: Warning: Failed to initialize \"%s\" backend, falling back to default GPU backend\n", __func__, backend_name);
+                }
             }
             if (!backend) {
                 backend = ggml_backend_init_by_type(GGML_BACKEND_DEVICE_TYPE_GPU, nullptr);