fixed cancellation for some worker tasks

In ShipModelGUI I faced a bug with the rename option on improve_shipgfx_export branch, where the rename was applied to the UI even though it failed. Then I checked the worker exception handling and found that it doesn't pass on any exceptions.

My first thought was I'd need to refactor it to interrupt the callee. Later I however found that the cancel option was affected too, and needs to be checked anyhow. Furthermore when you have broken files, on load even a partial result often is better than none.

Threrefore I opted to just cleanup some code, improve error reporting and fix the task cancellation.