[webcrypto] Reject failed operations with a DOMException rather than null.

LayoutTests/crypto/sign-verify-badParameters.html

Index: LayoutTests/crypto/sign-verify-badParameters.html
diff --git a/LayoutTests/crypto/sign-verify-badParameters.html b/LayoutTests/crypto/sign-verify-badParameters.html
index 252de91485635a33031dc755346eea4ded1597d1..458acd28dcff92680a159b02fff01f25fa2ae564 100644
--- a/LayoutTests/crypto/sign-verify-badParameters.html
+++ b/LayoutTests/crypto/sign-verify-badParameters.html
@@ -32,33 +32,28 @@ importHmacKey().then(function(result) {
     // Pass invalid signature parameters to verify()
     return crypto.subtle.verify(hmac, key, null, data);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     // Pass invalid signature parameters to verify()
     return crypto.subtle.verify(hmac, key, 'a', data);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     // Pass invalid signature parameters to verify()
     return crypto.subtle.verify(hmac, key, [], data);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     // Operation does not support signing.
     return crypto.subtle.sign({name: 'sha-1'}, key, data);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     // Operation doesn't support signing (also given an invalid key, but the
     // first failure takes priority)
     return crypto.subtle.sign({name: 'RSAES-PKCS1-v1_5'}, key, data);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 }).then(finishJSTest, failAndFinishJSTest);
 
 </script>