Improve exception messages in FolderWorkflowImpl

When org.hippoecm.repository.standardworkflow.FolderWorkflowImpl throws a WorkflowException, the message cannot be interpreted easy into a (configuration) solution for a project.

E.g. in #add() there is "No category defined for add to folder" and "No template defined for add to folder" which is not very descriptive and could have lot more information based on what data this exception is thrown.

Suggestions:
"Node hippo:configuration/hippo:queries/hippo:templates does not have subnode [category] defining a category for adding a [template] to a folder"

"The results from query [jcrStatement] at node [hippo:configuration/hippo:queries/hippo:templates/[category] did not contain a node named hipposysedit:prototype with type [template] of a node named [template]"