[indexeddb] Change IDBRequest.errorCode property to match new Exception type model

Jonas,

We're removing error code values as part of the new exception type model.  This will impact the IDBRequest.errorCode property.  I believe we want to rename this property to errorName and change its type to DOMString in order to match the new Exception type model name. This change will impact all the places where errorCode is used today in the spec.  However, it should be fairly easy to fix assuming we follow the above model.

Do you agree?

Israel