Command not found message should be configurable

[jvalkeal]

ThrowableResultHandler also handles CommandNotFound and thus we get rather long message which really is meant for last resort:

No command found for 'wrong'
org.springframework.shell.CommandNotFound: No command found for 'wrong'
        at org.springframework.shell.Shell.evaluate(Shell.java:205)
        at org.springframework.shell.Shell.run(Shell.java:158)
        at org.springframework.shell.jline.NonInteractiveShellRunner.run(NonInteractiveShellRunner.java:129)
        at org.springframework.shell.DefaultShellApplicationRunner.run(DefaultShellApplicationRunner.java:65)
        at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:761)
...

While it has always been possible to customise by adding a new bean something like:

public class CommandNotFoundResultHandler extends TerminalAwareResultHandler<CommandNotFound> {

	protected CommandNotFoundResultHandler(Terminal terminal) {
		super(terminal);
	}

	@Override
	protected void doHandleResult(CommandNotFound result) {
		terminal.writer().println("hello");
		terminal.writer().flush();
	}

}

It'd be nice we handle "command not found" centrally and make it easier configure and extend. We could have some kind of "context" what a new implementation could use to describe better error.

In 3.2.x we should make this default behaviour and in older versions allow user to enable it(don't want to change default behaviour).