[webcrypto] Match the error handling defined by the spec.

Source/modules/crypto/Key.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 39 matching lines @@
 
     ~Key();
 
     String type() const;
     bool extractable() const;
     Algorithm* algorithm();
     Vector<String> usages() const;
 
     const blink::WebCryptoKey& key() const { return m_key; }
 
-    bool canBeUsedForAlgorithm(const blink::WebCryptoAlgorithm&, AlgorithmOperation, ExceptionState&) const;
+    // If the key cannot be used with the indicated algorithm, returns false
+    // and fills the provided String with error details.
+    bool canBeUsedForAlgorithm(const blink::WebCryptoAlgorithm&, AlgorithmOperation, String&) const;
 
     static bool parseFormat(const String&, blink::WebCryptoKeyFormat&, ExceptionState&);
     static bool parseUsageMask(const Vector<String>&, blink::WebCryptoKeyUsageMask&, ExceptionState&);
 
 protected:
     explicit Key(const blink::WebCryptoKey&);
 
     const blink::WebCryptoKey m_key;
     RefPtr<Algorithm> m_algorithm;
 };
 
 } // namespace WebCore
 
 #endif