dawn_node: Error if the specified backend is not found

Defaulting to the 0'th adapter can silently use the Null adapter, which is no fun for anyone.

Bug tint:1354

Change-Id: I14e2379175cb90a48753bd81d096391d15d2dc22
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/75070
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_node/binding/GPU.cpp

@@ -112,6 +112,9 @@ namespace wgpu { namespace binding {
         std::transform(forceBackend.begin(), forceBackend.end(), forceBackend.begin(),
                        [](char c) { return std::tolower(c); });
 
+        // Default to first adapter if a backend is not specified
+        size_t adapterIndex = 0;
+
         if (!forceBackend.empty()) {
             if (forceBackend == "null") {
                 targetBackendType = wgpu::BackendType::Null;
@@ -129,19 +132,26 @@ namespace wgpu { namespace binding {
                 targetBackendType = wgpu::BackendType::OpenGL;
             } else if (forceBackend == "opengles" || forceBackend == "gles") {
                 targetBackendType = wgpu::BackendType::OpenGLES;
-            }
+            } else {
+                promise.Reject("unknown backend '" + forceBackend + "'");
+                return promise;
             }
 
-        // Default to first adapter if we don't find a match
+            bool found = false;
-        size_t adapterIndex = 0;
             for (size_t i = 0; i < adapters.size(); ++i) {
                 wgpu::AdapterProperties props;
                 adapters[i].GetProperties(&props);
                 if (props.backendType == targetBackendType) {
                     adapterIndex = i;
+                    found = true;
                     break;
                 }
             }
+            if (!found) {
+                promise.Reject("backend '" + forceBackend + "' not found");
+                return promise;
+            }
+        }
 
         auto adapter = GPUAdapter::Create<GPUAdapter>(env, adapters[adapterIndex], flags_);
         promise.Resolve(std::optional<interop::Interface<interop::GPUAdapter>>(adapter));