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

LayoutTests/crypto/exportKey-unextractable.html

Index: LayoutTests/crypto/exportKey-unextractable.html
diff --git a/LayoutTests/crypto/exportKey-unextractable.html b/LayoutTests/crypto/exportKey-unextractable.html
index 5992088ac90707acbe8dfd47c7902901c60aa7c3..903c5f65772475b5b75b43da5c462b260e0220fd 100644
--- a/LayoutTests/crypto/exportKey-unextractable.html
+++ b/LayoutTests/crypto/exportKey-unextractable.html
@@ -49,23 +49,19 @@ function importUnextractableRsaPrivateKey()
 importUnextractableAesKey().then(function(key) {
     return crypto.subtle.exportKey('raw', key);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     return importUnextractableHmacKey();
 }).then(function(key) {
     return crypto.subtle.exportKey('raw', key);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     return importUnextractableRsaPrivateKey();
 }).then(function(key) {
     return crypto.subtle.exportKey('pkcs8', key);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-
-    shouldBeNull("error");
+    logError(result);
 }).then(finishJSTest, failAndFinishJSTest);
 
 </script>