Improve error message in navigator.requestMediaKeySystemAccess()

Source/modules/encryptedmedia/NavigatorRequestMediaKeySystemAccess.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "modules/encryptedmedia/NavigatorRequestMediaKeySystemAccess.h"
 
 #include "bindings/core/v8/ScriptPromiseResolver.h"
 #include "bindings/core/v8/ScriptState.h"
 #include "core/dom/DOMException.h"
@@ skipping 175 matching lines @@
     }
 
 
     // 5. Let origin be the origin of document.
     //    (Passed with the execution context in step 7.)
 
     // 6. Let promise be a new promise.
     Document* document = toDocument(executionContext);
     if (!document->page()) {
         return ScriptPromise::rejectWithDOMException(
-            scriptState, DOMException::create(InvalidStateError, "Document does not have a page."));
+            scriptState, DOMException::create(InvalidStateError, "The context provided is not associated with a page."));
     }
 
     MediaKeySystemAccessInitializer* initializer = new MediaKeySystemAccessInitializer(scriptState, keySystem, supportedConfigurations);
     ScriptPromise promise = initializer->promise();
 
     // 7. Asynchronously determine support, and if allowed, create and
     //    initialize the MediaKeySystemAccess object.
     MediaKeysController* controller = MediaKeysController::from(document->page());
     WebEncryptedMediaClient* mediaClient = controller->encryptedMediaClient(executionContext);
     mediaClient->requestMediaKeySystemAccess(WebEncryptedMediaRequest(initializer));
 
     // 8. Return promise.
     return promise;
 }
 
 } // namespace blink