Init Flash error message on approval api errors.

app/assets/javascripts/merge_request_widget/approvals/approvals_api.js.es6

@@ -14,20 +14,29 @@
     }
 
     fetchApprovals() {
+      const flashErrorMessage =  'An error occured while retrieving approval data for this merge request.';
+
       return Vue.http.get(this.baseEndpoint).catch((err) => {
-        console.error(`Error fetching approvals. ${err}`);
+        console.error('Error fetching approvals', err);
+        return new Flash(flashErrorMessage, 'alert');
       });
     }
 
     approveMergeRequest() {
+      const flashErrorMessage = 'An error occured while submitting your approval.';
+
       return Vue.http.post(this.baseEndpoint).catch((err) => {
-        console.error(`Error approving merge request. ${err}`);
+        console.error('Error approving merge request', err);
+        return new Flash(flashErrorMessage, 'alert');
       });
     }
 
     unapproveMergeRequest() {
+      const flashErrorMessage = 'An error occured while removing your approval.';
+
       return Vue.http.delete(this.baseEndpoint).catch((err) => {
-        console.error(`Error unapproving merge request. ${err}`);
+        console.error('Error unapproving merge request', err);
+        return new Flash(flashErrorMessage, 'alert');
       });
     }
   }