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)
Objects are always passed by reference, so it doesn't make sense to
return an object which is passed by reference as it will always be the
same object. This change makes the code a bit more readable.
Instead of just say “Failed to save entry” we’ll save the entry at all cost and try to fetch content. If fetching content failed, the entry will still be saved at least, but without content.
At the moment only Readability & wallabag v2 import allow created_at import.
Pocket removed `time_added` field from their API v2 to v3...
And wallabag v1 doesn't export that value.
graby will throw an Exception in some case (like a bad url, a restricted url or a secured pdf).
Import doesn't handle that case and break the whole import.
With that commit the import isn't stopped but the entry is just skipped.
Also, as a bonus, I've added extra test on WallabagImportV2 when the json is empty.
