Structured cloning: improve DataCloneError reporting.

LayoutTests/storage/indexeddb/objectstore-basics-expected.txt

 Test the basics of IndexedDB's IDBObjectStore.
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 indexedDB = self.indexedDB || self.webkitIndexedDB || self.mozIndexedDB || self.msIndexedDB || self.OIndexedDB;
 
 dbname = "objectstore-basics.html"
 indexedDB.deleteDatabase(dbname)
 indexedDB.open(dbname)
@@ skipping 79 matching lines @@
 addData():
 db = event.target.result
 transaction = db.transaction(['storeName'], 'readwrite')
 store = transaction.objectStore('storeName')
 Try to insert data with a Date key:
 store.add({x: 'foo'}, testDate)
 Try to insert a value not handled by structured clone:
 Expecting exception from store.add({x: 'bar', y: self}, 'bar')
 PASS Exception was thrown.
 PASS code is DOMException.DATA_CLONE_ERR
-Exception message: An object could not be cloned.
+Exception message: Failed to execute 'add' on 'IDBObjectStore': An object could not be cloned.
 Try to insert data where key path yields a Date key:
 store.add({x: testDateB, y: 'value'}, 'key')
 addSuccess():
 PASS event.target.result is "key"
 event.target.source.add({x: 'foo'}, 'zzz')
 addAgainFailure():
 PASS event.target.error.name is 'ConstraintError'
 event.preventDefault()
 db.transaction(['storeName'], 'readwrite')
 store = transaction.objectStore('storeName')
@@ skipping 117 matching lines @@
 The key parameter was provided but does not contain a valid key.
 Expecting exception from storeWithOutOfLineKeys.add({}, null)
 PASS Exception was thrown.
 PASS code is 0
 PASS ename is 'DataError'
 Exception message: Failed to execute 'add' on 'IDBObjectStore': The parameter is not a valid key.
 PASS successfullyParsed is true
 
 TEST COMPLETE