[IndexedDB] there should not be a message exception field on IDBDatabaseException

[Cameron McCormack]

Web IDL forbids exception fields named "message" (and "name").  This 
property will exist automatically as an own property on the exception 
object if a message is specified when throwing the exception.  So you 
just need to say, for example

  Throw an IDBDatabaseException of type "VersionError" with message "Get
  with the times!".