throw key errors before download begins

frontend/src/fileReceiver.js

@@ -7,8 +7,23 @@ class FileReceiver extends EventEmitter {
   }
 
   download() {
-    return Promise.all([
+    return window.crypto.subtle
-      new Promise((resolve, reject) => {
+      .importKey(
+        'jwk',
+        {
+          kty: 'oct',
+          k: location.hash.slice(1),
+          alg: 'A128GCM',
+          ext: true
+        },
+        {
+          name: 'AES-GCM'
+        },
+        true,
+        ['encrypt', 'decrypt']
+      )
+      .then(key => {
+        return new Promise((resolve, reject) => {
           const xhr = new XMLHttpRequest();
 
           xhr.onprogress = event => {
@@ -27,12 +42,15 @@ class FileReceiver extends EventEmitter {
             const fileReader = new FileReader();
             fileReader.onload = function() {
               const meta = JSON.parse(xhr.getResponseHeader('X-File-Metadata'));
-            resolve({
+              resolve([
+                {
                   data: this.result,
                   aad: meta.aad,
                   filename: meta.filename,
                   iv: meta.id
-            });
+                },
+                key
+              ]);
             };
 
             fileReader.readAsArrayBuffer(blob);
@@ -41,22 +59,8 @@ class FileReceiver extends EventEmitter {
           xhr.open('get', '/assets' + location.pathname.slice(0, -1), true);
           xhr.responseType = 'blob';
           xhr.send();
-      }),
+        });
-      window.crypto.subtle.importKey(
+      })
-        'jwk',
-        {
-          kty: 'oct',
-          k: location.hash.slice(1),
-          alg: 'A128GCM',
-          ext: true
-        },
-        {
-          name: 'AES-GCM'
-        },
-        true,
-        ['encrypt', 'decrypt']
-      )
-    ])
       .then(([fdata, key]) => {
         this.emit('decrypting', true);
         return Promise.all([