Simplify controllers code.

Return null (or mark method as returning void) when view name may be
deduced from request URL.

Inspired by:
http://stackoverflow.com/q/6875255/what-does-it-mean-when-spring-mvc-controller-returns-null-view-name

No functional changes.

Author: php-coder

src/main/java/ru/mystamps/web/controller/ActivateAccountController.java

@@ -82,7 +82,7 @@ public String processInput(
 			final BindingResult result) {
 
 		if (result.hasErrors()) {
-			return "account/activate";
+			return null;
 		}
 
 		userService.registerUser(

src/main/java/ru/mystamps/web/controller/AuthAccountController.java

@@ -97,7 +97,7 @@ public String processInput(
 				siteService.logAboutFailedAuthentication(page, user, ip, referer, agent);
 			}
 
-			return "account/auth";
+			return null;
 		}
 
 		final User user = userService.findByLogin(form.getLogin());

src/main/java/ru/mystamps/web/controller/NotFoundErrorController.java

@@ -48,7 +48,7 @@ public class NotFoundErrorController {
 	}
 
 	@RequestMapping(method = RequestMethod.GET)
-	public String notFound(
+	public void notFound(
 			final HttpServletRequest request,
 			final HttpSession session,
 			@RequestHeader(value = "referer", required = false) final String referer,
@@ -66,8 +66,6 @@ public String notFound(
 			// database error should not break showing of 404 page
 			log.warn("Cannot log 404 error", ex);
 		}
-		
-		return "error/404";
 	}
 
 }

src/main/java/ru/mystamps/web/controller/RegisterAccountController.java

@@ -69,7 +69,7 @@ public String processInput(
 			final BindingResult result) {
 
 		if (result.hasErrors()) {
-			return "account/register";
+			return null;
 		}
 
 		userService.addRegistrationRequest(form.getEmail());