[ware]Avoid an error when folderId is null at getCheckOutDocs

Until now, the root folder is happened to be returned when folderId is
null. This depends on the implementation.
[ware]Output a correct error message at checkAddProperty
[ware]Change changeToken type from long to String

changeToken value is still sequential number, but its type is now
String. This is for another back end that doesn't use numeric
changeToken.