full unit testing and fixes

- mapToString is ordered according to setup
- setupSingle & teardownSingle no longer return "ok" states
- teardown functions support errors
- setup and teardown functions receive module

module.go

@@ -1,17 +1,24 @@
 package app
 
 import (
+	"errors"
 	"fmt"
 	"log/slog"
 )
 
+// ErrModuleNotFound is returned when a module is not found in the lifecycle.
+var ErrModuleNotFound = errors.New("module not found")
+
+// moduleFn is a function type for module setup and teardown functions.
+type moduleFn func(*Module) error
+
 // Module represents a sub-system of the application, with its setup and
 // teardown functions and dependencies.
 type Module struct {
 	logger   *slog.Logger
 	name     string
-	setup    setupFn
-	teardown teardownFn
+	setup    moduleFn
+	teardown moduleFn
 	depends  []string
 	loaded   bool // loaded indicates if the module has been set up
 }
@@ -19,9 +26,9 @@ type Module struct {
 // ModuleOpts contains user-exposed options when defining a module.
 type ModuleOpts struct {
 	// Setup is the setup function for the module.
-	Setup setupFn
+	Setup moduleFn
 	// Teardown is the teardown function for the module.
-	Teardown teardownFn
+	Teardown moduleFn
 	// Depends is a list of modules that this module depends on. Each entry must
 	// be the exact name of a module registered in the lifecycle.
 	Depends []string