fix: handle fetch errors

src/fallbackServiceStub.ts

@@ -171,7 +171,8 @@ export function generateServiceStub(
                 }
               });
           }
-        });
+        })
+        .catch((err: unknown) => callback(err));
 
       if (rpc.responseStream) {
         return streamArrayParser;

test/unit/grpc-fallback.ts

@@ -284,7 +284,7 @@ describe('grpc-fallback', () => {
     });
   });
 
-  it('should handle an error', done => {
+  it('should handle an API error', done => {
     const requestObject = {content: 'test-content'};
     // example of an actual google.rpc.Status error message returned by Language API
     const fixtureName = path.resolve(__dirname, '..', 'fixtures', 'error.bin');
@@ -327,6 +327,18 @@ describe('grpc-fallback', () => {
     });
   });
 
+  it('should handle a fetch error', done => {
+    const requestObject = {content: 'test-content'};
+    //@ts-ignore
+    sinon.stub(nodeFetch, 'Promise').rejects(new Error('fetch error'));
+    gaxGrpc.createStub(echoService, stubOptions).then(echoStub => {
+      echoStub.echo(requestObject, {}, {}, (err?: Error) => {
+        assert.strictEqual(err?.message, 'fetch error');
+        done();
+      });
+    });
+  });
+
   it('should promote ErrorInfo if exist in fallback-rest error', done => {
     const requestObject = {content: 'test-content'};
     // example of an actual google.rpc.Status error message returned by Translate API