Do not print error for user interrupted script

Karaf allows for the usage of commands like `log:tail` in `etc/shell.init.script` which is handy for example for a development assembly. But if such a command is canceled by a user (e.g. by hitting Ctrl+C) an error message is logged and printed to stderr, claiming an error in the initialization script.

While the debug logging might still be handy, it would be nice to not have the stderr message on user interrupts.