added error handling

backend/app.js

@@ -109,7 +109,7 @@ function getPairs(callback){
      if(err) {
        // Handle error
        console.log(err);
-       return null;
+       callback(null, true);
      } else {
        const pairs = [];
        
@@ -126,7 +126,7 @@ function getPairs(callback){
           pairs.push({name1:name1,name2:name2});
          }
        }
-       callback(pairs);
+       callback(pairs,false);
      }
     });
 }
@@ -143,7 +143,7 @@ function getAvailableNames(callback){
    if(err) {
      // Handle error
      console.log(err);
-     return null;
+     callback(null, true);
    } else {
      const names = [];
      
@@ -159,7 +159,7 @@ function getAvailableNames(callback){
       }
       
      }
-     callback(names);
+     callback(names,false);
    }
   });
 }
@@ -218,8 +218,18 @@ function count_pairs(row, col, cntr){
 }
 
 function MakePairsV3(callback){
-  executeAPI(getAvailableNames,function(names){
-    executeAPI(getPairs, function(pairs){
+  executeAPI(getAvailableNames,function(names,error){
+    if (error){
+      console.log("Error - could not load names");
+      callback.send({pairs: null, left: null, error: true, error_message: 'Could not load names list'});
+      return;
+    }
+    executeAPI(getPairs, function(pairs,error){
+      if (error){
+        console.log("Error - could not load pairs");
+        callback.send({pairs: null, left: null, error: true, error_message: 'Could not load pairs list'});
+        return;
+      }
       max_pairs = Math.floor(names.length/2);
       matrix=[];
 
@@ -244,8 +254,6 @@ function MakePairsV3(callback){
         }
       });
 
-      
-
       tree = [];
       done=false;
 
@@ -289,9 +297,10 @@ function MakePairsV3(callback){
 
       if (names.length === 0) names=null;
       pairsForSave = result_pairs;
-      callback.send({pairs: result_pairs, left: names});
+      callback.send({pairs: result_pairs, left: names, error:false});
     });
   });
+  
 }
 
 function SavePairs(callback){

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>