Validate imported entry to avoid error on import

We got some imports with a missing `url` field generating some errors while trying to retrieve an existing entry with that url.
Introducing the `validateEntry` allow us to dismiss a message when it doesn't have an url (or other missing stuff in the future)

src/Wallabag/ImportBundle/Consumer/AbstractConsumer.php

@@ -52,6 +52,13 @@ abstract class AbstractConsumer
 
         $this->import->setUser($user);
 
+        if (false === $this->import->validateEntry($storedEntry)) {
+            $this->logger->warning('Entry is invalid', ['entry' => $storedEntry]);
+
+            // return true to skip message
+            return true;
+        }
+
         $entry = $this->import->parseEntry($storedEntry);
 
         if (null === $entry) {