IndexedDB: Log exception messages

LayoutTests/storage/indexeddb/mozilla/clear-expected.txt

 Test IndexedDB's clearing an object store
 
 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 = "clear.html"
 indexedDB.deleteDatabase(dbname)
 indexedDB.open(dbname)
 objectStore = db.createObjectStore('foo', { autoIncrement: true });
 request = objectStore.add({});
 Expecting exception from db.transaction('foo').objectStore('foo').clear();
 PASS Exception was thrown.
 PASS code is 0
 PASS ename is 'ReadOnlyError'
+Exception message: A write operation was attempted in a read-only transaction.
 db.transaction('foo', 'readwrite')
 transaction.objectStore('foo').clear();
 request = db.transaction('foo').objectStore('foo').openCursor();
 cursor = request.result;
 PASS cursor is null
 PASS successfullyParsed is true
 
 TEST COMPLETE