App enters in `UnsuccesfulResponseException` notification loop

# Issue details

### Duplicate?
Have you searched the issues of this repository if your issue is already known? yes

### Actual behaviour

I tried to add an entry through the app, but my wallabag server crashed (error 500) because it could not insert the entry in the database due to the entry containing emojis and the database charset not being properly configured.

The app enters in a notification loop:

> Error: com.di77n.stuff.wallabag.apiwrapper.exceptions.UnsuccesfulResponseException: HTTP response: 500

Every few seconds the app retry to add the entry and re triggers the notification. This is really annoying.

The only way to make it stop is to force stop the app from the android settings.

The app becomes unusable as long as the server is not fixed.

### Expected behaviour

There should be a way to cancel an entry.

# Steps to reproduce the issue

1. Have a badly configured server (`database_charset: utf8` instead of `database_charset: utf8mb4`)
2. Add an entry that crash the server (entry with emojis [examle](https://github.com/simple-login/app/blob/master/README.md))

# Environment details
- **wallabag app version**: 2.3.0
- **wallabag app installation source** (e.g. Gplay, F-Droid, manual): F-Droid
- **Android OS version**: 9
- **Android ROM**: OxygenOS 9.0.10
- **Android hardware**: OnePlus 5
- **wallabag server version**: 2.3.8
- **Do you have Two-Factor-Authentication enabled?**: yes/no

# Logs
## wallabag server
Please paste relevant wallabag server logs here (from file `/var/logs/prod.log`):

Just any CRITICAL error log.

## Web server
Please paste relevant web server logs (e.g. from nginx, Apache, …) here:

```
172.18.0.9 - - [19/Jan/2020:15:56:59 +0100] "POST /api/entries.json HTTP/1.1" 500 83 "-" "okhttp/4.2.2"
```

# Your experience with wallabag Android app
Have you had any luck using wallabag Android app before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

Can't live without it, wallabag would be unusable on mobile without it.