added error handling

web/src/App.js

@@ -21,7 +21,7 @@ class App extends Component {
     this.setState({waitingPairs:true, renderFinish:false, waitingSave:true});
     
     fetch(url, {}).then(function(response) { return response.json(); }).then(function(data) {
-      this.setState({pairs : data.pairs, left:data.left, renderPairsList:true, waitingPairs:false, waitingSave:false});
+      this.setState({pairs : data.pairs, left:data.left, renderPairsList:!data.error, waitingPairs:false, waitingSave:false, error: data.error, error_message: data.error_message});
     }.bind(this));
 
 
@@ -46,6 +46,12 @@ class App extends Component {
         <div className = "horizontalDiv">
           <button disabled={this.state.waitingPairs} onClick = {this.getPairsEventHandler}>Get pairs</button>
         </div>
+        {
+          this.state.error &&
+          <div>
+            <h2> Error - {this.state.error_message} </h2>
+          </div>
+        }
         {
           this.state.renderPairsList &&
           <div>