Improved error reporting: Invalid working directory

The installscript may have bugs and figuring out those from logs can be time consuming.

Operation \"Execute\" with arguments \"workingdirectory=/home/<user>/some/path/
...
failed: Execution failed (Unexpected exit code: 255)

The IFW could sanity check that the "workingdirectory" actually exists. If not then mention that in the UI error dialog and in the command line error text.

Similarly it could report if the file it tries to execute is missing.