Better error message for unknown commands (#2311)

Sources/ContainerCommands/DefaultCommand.swift

@@ -84,12 +84,11 @@ struct DefaultCommand: AsyncLoggableCommand {
         guard let plugin = pluginLoader?.findPlugin(name: command), plugin.config.isCLI else {
             throw ValidationError(
                 """
-                Plugin 'container-\(command)' not found.
+                unknown command '\(command)'
 
                 - If system services are not running, start them with: container system start
-                - If the plugin isn't installed, ensure it exists under:
 
-                Check to see that the plugin exists under:
+                If '\(command)' is a plugin, check that it exists under:
                   - \(hintPaths)
 
                 """