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

Source/modules/crypto/Key.cpp

 /*
  * 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 157 matching lines @@
 {
     Vector<String> result;
     for (size_t i = 0; i < WTF_ARRAY_LENGTH(keyUsageMappings); ++i) {
         blink::WebCryptoKeyUsage usage = keyUsageMappings[i].value;
         if (m_key.usages() & usage)
             result.append(keyUsageToString(usage));
     }
     return result;
 }
 
-bool Key::canBeUsedForAlgorithm(const blink::WebCryptoAlgorithm& algorithm, AlgorithmOperation op, ExceptionState& exceptionState) const
+bool Key::canBeUsedForAlgorithm(const blink::WebCryptoAlgorithm& algorithm, AlgorithmOperation op, String& errorDetails) const
 {
     if (!(m_key.usages() & toKeyUsage(op))) {
-        exceptionState.throwDOMException(NotSupportedError, "key.usages does not permit this operation");
+        errorDetails = "key.usages does not permit this operation";
         return false;
     }
 
     if (m_key.algorithm().id() != algorithm.id()) {
-        exceptionState.throwDOMException(NotSupportedError, "key.algorithm does not match that of operation");
+        errorDetails = "key.algorithm does not match that of operation";
         return false;
     }
 
     // Verify that the algorithm-specific parameters for the key conform to the
     // algorithm.
-    // FIXME: Verify that this is complete.
+    // FIXME: This is incomplete and not future proof. Operational parameters
+    //        should be enumerated when defining new parameters.
 
     if (m_key.algorithm().id() == blink::WebCryptoAlgorithmIdHmac) {
         blink::WebCryptoAlgorithmId keyHash;
         blink::WebCryptoAlgorithmId algorithmHash;
         if (!getHmacHashId(m_key.algorithm(), keyHash) || !getHmacHashId(algorithm, algorithmHash) || keyHash != algorithmHash) {
-            exceptionState.throwDOMException(NotSupportedError, "key.algorithm does not match that of operation (HMAC's hash differs)");
+            errorDetails = "key.algorithm does not match that of operation (HMAC's hash differs)";
             return false;
         }
     }
 
     return true;
 }
 
 bool Key::parseFormat(const String& formatString, blink::WebCryptoKeyFormat& format, ExceptionState& exceptionState)
 {
     // There are few enough values that testing serially is fast enough.
@@ skipping 26 matching lines @@
         if (!usage) {
             exceptionState.throwTypeError("Invalid keyUsages argument");
             return false;
         }
         mask |= usage;
     }
     return true;
 }
 
 } // namespace WebCore