Make <<>> fail if it cannot open a file

Make <<>> fail if it cannot open a file

By default, only a warning is printed and the script continues.

Example of this situation:

    % novaboot non-existent --gen-only
    novaboot: Read /home/wsh/.config/novaboot
    novaboot: Effective options: non-existent --gen-only
    Can't open non-existent: No such file or directory at /home/wsh/src/novaboot/novaboot line 362.

We do not add a test for this, because we make all warnings fatal when
running tests.