log errors storing persistance

lib/Persistence/PurgeLimiter.php

@@ -73,7 +73,10 @@ class PurgeLimiter extends AbstractPersistence
         if ($pl + self::$_limit >= $now) {
             return false;
         }
-        self::$_store->setValue((string) $now, 'purge_limiter');
-        return true;
+        $hasStored = self::$_store->setValue((string) $now, 'purge_limiter');
+        if (!$hasStored) {
+            error_log('failed to store the purge limiter, skipping purge cycle to avoid getting stuck in a purge loop');
+        }
+        return $hasStored;
     }
 }

lib/Persistence/ServerSalt.php

@@ -65,7 +65,9 @@ class ServerSalt extends AbstractPersistence
             self::$_salt = $salt;
         } else {
             self::$_salt = self::generate();
-            self::$_store->setValue(self::$_salt, 'salt');
+            if (!self::$_store->setValue(self::$_salt, 'salt')) {
+                error_log('failed to store the server salt, delete tokens, traffic limiter and user icons won\'t work');
+            }
         }
         return self::$_salt;
     }

lib/Persistence/TrafficLimiter.php

@@ -172,7 +172,9 @@ class TrafficLimiter extends AbstractPersistence
             $tl     = time();
             $result = true;
         }
-        self::$_store->setValue((string) $tl, 'traffic_limiter', $hash);
+        if (!self::$_store->setValue((string) $tl, 'traffic_limiter', $hash)) {
+            error_log('failed to store the traffic limiter, it probably contains outdated information');
+        }
         return $result;
     }
 }