Implement more flexible error handling

- Add exception handling around new interface CommandExceptionResolver
  which allows to define a chain of resolvers to process errors before
  exception is bubbled up to result handlers.
- Will be foundation to add more sophisticated error handling features
  compared to what spring itself have for rest layer.
- Resolver returns CommandHandlingResult holder which further can be
  used to make a choice what to print into console and if spesific exit
  code should be used in non-interactive mode.
- Exception handling can be defined globally and per command giving
  a change for user to customise i.e. error thrown by parser.
- CommandParserExceptionResolver replaces CommandParserExceptionsExceptionResultHandler
  and provides more meaninful message for missing options.
- Fixes #503

spring-shell-autoconfigure/src/main/java/org/springframework/shell/boot/ExitCodeAutoConfiguration.java

@@ -20,11 +20,13 @@ import java.util.List;
 import java.util.function.Function;
 
 import org.springframework.boot.ExitCodeExceptionMapper;
+import org.springframework.boot.ExitCodeGenerator;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.shell.command.CommandExecution;
+import org.springframework.shell.exit.ExitCodeExceptionProvider;
 import org.springframework.shell.exit.ExitCodeMappings;
 
 /**
@@ -47,6 +49,12 @@ public class ExitCodeAutoConfiguration {
 		return new ShellExitCodeMappingsExceptionMapper();
 	}
 
+	@Bean
+	@ConditionalOnMissingBean
+	public ExitCodeExceptionProvider exitCodeExceptionProvider() {
+		return (exception, code) -> new ShellExitCodeException(exception, code);
+	}
+
 	static class ShellExitCodeExceptionMapper implements ExitCodeExceptionMapper {
 
 		@Override
@@ -85,4 +93,19 @@ public class ExitCodeAutoConfiguration {
 			return exitCode;
 		}
 	}
+
+	static class ShellExitCodeException extends RuntimeException implements ExitCodeGenerator {
+
+		private int code;
+
+		ShellExitCodeException(Throwable throwable, int code) {
+			super(throwable);
+			this.code = code;
+		}
+
+		@Override
+		public int getExitCode() {
+			return code;
+		}
+	}
 }