Improve help for subcommands with an executable handler

- Support subcommands with only a short help flag
- Get help flags from the subcommand (enables additional customization)
- Ensures consistency with how help for subcommands without an
executable handler is implemented (preSubcommand hooks are not called)

lib/command.js

@@ -1096,14 +1096,16 @@ Expecting one of '${allowedValues.join("', '")}'`);
       this.help();
     }
     const subCommand = this._findCommand(subcommandName);
-    if (subCommand && !subCommand._executableHandler) {
+    if (!subCommand) {
+      this.help({ error: true });
+    } else if (!subCommand._executableHandler) {
       subCommand.help();
     }
 
     // Fallback to parsing the help flag to invoke the help.
-    if (this._helpLongFlag) {
-      return this._dispatchSubcommand(subcommandName, [], [this._helpLongFlag]);
-    }
+    this._executeSubCommand(subCommand, [
+      subCommand._helpLongFlag || subCommand._helpShortFlag
+    ]);
   }
 
   /**