chore(docs): add an Error handling sample

samples/error.js

@@ -0,0 +1,52 @@
+/**
+ * Copyright 2019 Google LLC.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+// sample-metadata
+//  title: error sample
+//  description: sample show how to handle and process error
+//  usage: node error.js
+
+const {Datastore} = require('@google-cloud/datastore');
+
+// [START error]
+function main() {
+  // Creates a client
+  const datastore = new Datastore();
+
+  const query = datastore.createQuery(['Company']).start('badrequest');
+
+  async function runQuery() {
+    try {
+      const [result] = await datastore.runQuery(query);
+      // etc., etc.
+      return result;
+    } catch (error) {
+      // do something with error.
+      console.log(error.code); // 3
+      //Forward the error to caller
+      throw error;
+    }
+  }
+
+  runQuery().catch(err => {
+    console.log(err.message); // "Error parsing protocol message"
+  });
+  // [END error]
+}
+
+main(...process.argv.slice(2));

samples/test/error.test.js

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+/**
+ * Copyright 2019 Google LLC.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+const assert = require('assert');
+const {execSync} = require('child_process');
+const exec = cmd =>
+  execSync(cmd, {
+    encoding: 'utf8',
+  });
+
+describe('error', () => {
+  it('should have an error', done => {
+    const errorCode = 3;
+    const output = exec(`node error.js`);
+    assert.strictEqual(parseInt(output[0]), errorCode);
+    done();
+  });
+});