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-docs/src/main/asciidoc/using-shell-commands-exceptionhandling.adoc

@@ -1,5 +1,5 @@
 [[dynamic-command-exitcode]]
-=== Exit Code
+=== Exception Handling
 ifndef::snippets[:snippets: ../../test/java/org/springframework/shell/docs]
 
 Many command line applications when applicable return an _exit code_ which running environment
@@ -7,6 +7,44 @@ can use to differentiate if command has been executed successfully or not. In a
 this mostly relates when a command is run on a non-interactive mode meaning one command
 is always executed once with an instance of a `spring-shell`.
 
+==== Exception Resolving
+
+Unhandled exceptions will bubble up into shell's `ResultHandlerService` and then eventually
+handled by some instance of `ResultHandler`. Chain of `ExceptionResolver` implementations
+can be used to resolve exceptions and gives you flexibility to return message to get written
+into console together with exit code which are wrapped within `CommandHandlingResult`.
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/ErrorHandlingSnippets.java[tag=my-exception-resolver-class]
+----
+====
+
+`CommandExceptionResolver` implementations can be defined globally as beans or defined
+per `CommandRegistration` if it's applicable only for a particular command itself.
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/ErrorHandlingSnippets.java[tag=example1]
+----
+====
+
+Use you own exception types which can also be an instance of boot's `ExitCodeGenerator` if
+you want to define exit code there.
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/ErrorHandlingSnippets.java[tag=my-exception-class]
+----
+====
+
+NOTE: With annotation based configuration exception resolving can only be customised globally
+
+==== Exit Code Mappings
+
 Default behaviour of an exit codes is as:
 
 - Errors from a command option parsing will result code of `2`
@@ -37,3 +75,4 @@ include::{snippets}/ExitCodeSnippets.java[tag=example1]
 ====
 
 NOTE: Exit codes cannot be customized with annotation based configuration
+

spring-shell-docs/src/main/asciidoc/using-shell-commands.adoc

@@ -19,4 +19,4 @@ include::using-shell-commands-organize.adoc[]
 
 include::using-shell-commands-availability.adoc[]
 
-include::using-shell-commands-exitcode.adoc[]
+include::using-shell-commands-exceptionhandling.adoc[]

spring-shell-docs/src/test/java/org/springframework/shell/docs/ErrorHandlingSnippets.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.shell.docs;
+
+import org.springframework.shell.command.CommandRegistration;
+import org.springframework.shell.command.CommandExceptionResolver;
+import org.springframework.shell.command.CommandHandlingResult;
+
+class ErrorHandlingSnippets {
+
+	// tag::my-exception-class[]
+	static class CustomException extends RuntimeException {}
+	// end::my-exception-class[]
+
+	// tag::my-exception-resolver-class[]
+	static class CustomExceptionResolver implements CommandExceptionResolver {
+
+		@Override
+		public CommandHandlingResult resolve(Exception e) {
+			if (e instanceof CustomException) {
+				return CommandHandlingResult.of("Hi, handled exception\n", 42);
+			}
+			return null;
+		}
+	}
+	// end::my-exception-resolver-class[]
+
+	void dump1() {
+		// tag::example1[]
+		CommandRegistration.builder()
+			.withErrorHandling()
+				.resolver(new CustomExceptionResolver())
+				.and()
+			.build();
+		// end::example1[]
+	}
+
+}