Exclude ExitRequest from error handling

- Don't try to handle ExitRequest exception with error handling
  so that we're able to break out from a run loop with
  interactive shell.
- Fixes #624

spring-shell-core/src/main/java/org/springframework/shell/Shell.java

@@ -265,7 +265,7 @@ public class Shell {
 		finally {
 			Signals.unregister("INT", sh);
 		}
-		if (e != null) {
+		if (e != null && !(e instanceof ExitRequest)) {
 			try {
 				CommandHandlingResult processException = processException(commandExceptionResolvers, e);
 				processExceptionNonInt = processException;