When saving via postMessage (Action_Save) you get a response via Action_Save_Resp, which contains a "success" parameter. This parameter is always true, even if saving in the WOPI host failed. The parameter should reflect whether saving was successful or not. Additionally, delivering the error string reported by the WOPI host, would also be nice.
Moving to NEW
Samuel Mehrbrodt committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/online/commit/665b1629de30a4a402c6b10dd542de158db1f428 tdf#131123 Report back save result
Samuel Mehrbrodt committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/online/commit/9f8fdb7bd7b12c1591e2c118e59ea3154844c960 tdf#131123 Report back real save result
Samuel Mehrbrodt committed a patch related to this issue. It has been pushed to "libreoffice-7-0": https://git.libreoffice.org/online/commit/59d121bd26ce0a03fc5ec962faee65ec85f2d0ed tdf#131123 Report back save result