Preferences: Fix error messages and convenience configuration

refs #4648

config/preferences/KEEP.md

@@ -1 +0,0 @@
-# Keep this directory

library/Icinga/Application/Web.php

@@ -249,6 +249,15 @@ class Web extends ApplicationBootstrap
             $preferences->attach($sessionStore);
 
             if ($this->getConfig()->preferences !== null) {
+                if (!$this->getConfig()->preferences->type) {
+                    Logger::info(
+                        'Preferences provider configuration error. No type was omitted. For convenience we enable '
+                        . 'file based ini provider for you.'
+                    );
+
+                    $this->getConfig()->preferences->type = 'ini';
+                }
+
                 $path = Config::resolvePath($this->getConfig()->preferences->configPath);
                 if (is_dir($path) === false) {
                     Logger::error(

library/Icinga/User/Preferences/StoreFactory.php

@@ -28,11 +28,12 @@
 
 namespace Icinga\User\Preferences;
 
-use Icinga\User;
-use Icinga\Exception\ProgrammingError;
-use \Icinga\Application\DbAdapterFactory;
 use \Zend_Config;
 use \Zend_Db;
+use \Icinga\User;
+use \Icinga\Exception\ProgrammingError;
+use \Icinga\Application\DbAdapterFactory;
+use Icinga\Application\Logger;
 
 /**
  * Create preference stores from zend config
@@ -66,7 +67,10 @@ final class StoreFactory
             $store = new $class();
 
             if (!$store instanceof FlushObserverInterface) {
-                throw new ProgrammingError('Not instance of FlushObserverInterface: '. $class);
+                throw new ProgrammingError(
+                    'Preferences StoreFactory could not create provider (class ' . $class . '). '
+                    . 'Class is not instance of FlushObserverInterface.'
+                );
             }
 
             $items = $config->toArray();
@@ -88,6 +92,8 @@ final class StoreFactory
             return $store;
         }
 
-        throw new ProgrammingError('Could not instantiate class: '. $class);
+        throw new ProgrammingError(
+            'Preferences StoreFactory could not create provider (class ' . $class . '). Class not found.'
+        );
     }
 }