RPC_CondCabling: remove usage of DISP macros

• replace DISP macros with either a MsgStream (debug/verbose) or REPORT_MESSAGE (error)
• change various "error" print functions to return a std::string and mark them as nodiscard to ensure all code is migrated
• replace obsolete __osstream typedef with std::ostringstream

Relates to ATLASRECTS-6313.