Add more descriptive error message on XrdCl::CopyProcess:run

[andrea-manzi]

In case of errors when running a XrdCl::CopyProcess:Run it would be nice to add more descriptive messages.
For instance if TPC is not supported by either the source or the destination we get: [ERROR] Operation not supported, while it would be nice to have also reported what is not supported and which server we are talking about. Same for [ERROR] Operation expired.
thanks
Andrea