throw key errors before download begins

frontend/src/fileReceiver.js

@@ -7,42 +7,8 @@ class FileReceiver extends EventEmitter {
   }
 
   download() {
-    return Promise.all([
+    return window.crypto.subtle
-      new Promise((resolve, reject) => {
+      .importKey(
-        const xhr = new XMLHttpRequest();
-
-        xhr.onprogress = event => {
-          if (event.lengthComputable && event.target.status !== 404) {
-            this.emit('progress', [event.loaded, event.total]);
-          }
-        };
-
-        xhr.onload = function(event) {
-          if (xhr.status === 404) {
-            reject(new Error('notfound'));
-            return;
-          }
-
-          const blob = new Blob([this.response]);
-          const fileReader = new FileReader();
-          fileReader.onload = function() {
-            const meta = JSON.parse(xhr.getResponseHeader('X-File-Metadata'));
-            resolve({
-              data: this.result,
-              aad: meta.aad,
-              filename: meta.filename,
-              iv: meta.id
-            });
-          };
-
-          fileReader.readAsArrayBuffer(blob);
-        };
-
-        xhr.open('get', '/assets' + location.pathname.slice(0, -1), true);
-        xhr.responseType = 'blob';
-        xhr.send();
-      }),
-      window.crypto.subtle.importKey(
         'jwk',
         {
           kty: 'oct',
@@ -56,7 +22,45 @@ class FileReceiver extends EventEmitter {
         true,
         ['encrypt', 'decrypt']
       )
-    ])
+      .then(key => {
+        return new Promise((resolve, reject) => {
+          const xhr = new XMLHttpRequest();
+
+          xhr.onprogress = event => {
+            if (event.lengthComputable && event.target.status !== 404) {
+              this.emit('progress', [event.loaded, event.total]);
+            }
+          };
+
+          xhr.onload = function(event) {
+            if (xhr.status === 404) {
+              reject(new Error('notfound'));
+              return;
+            }
+
+            const blob = new Blob([this.response]);
+            const fileReader = new FileReader();
+            fileReader.onload = function() {
+              const meta = JSON.parse(xhr.getResponseHeader('X-File-Metadata'));
+              resolve([
+                {
+                  data: this.result,
+                  aad: meta.aad,
+                  filename: meta.filename,
+                  iv: meta.id
+                },
+                key
+              ]);
+            };
+
+            fileReader.readAsArrayBuffer(blob);
+          };
+
+          xhr.open('get', '/assets' + location.pathname.slice(0, -1), true);
+          xhr.responseType = 'blob';
+          xhr.send();
+        });
+      })
       .then(([fdata, key]) => {
         this.emit('decrypting', true);
         return Promise.all([