append command does not abort if "fromFile" is missing (#623)
append command does not abort if "fromFile" is missing (#623)

This is consistent with the behavior when using shell commands.
The return value now only controls whether the error message is
displayed or not.
OpenVnmrJ/OpenVnmrJ,OpenVnmrJ/OpenVnmrJ,OpenVnmrJ/OpenVnmrJ,OpenVnmrJ/OpenVnmrJ,OpenVnmrJ/OpenVnmrJ,OpenVnmrJ/OpenVnmrJ,OpenVnmrJ/OpenVnmrJ,OpenVnmrJ/OpenVnmrJ,OpenVnmrJ/OpenVnmrJ,OpenVnmrJ/OpenVnmrJ