Improve error notification text: HTTP response: 500 Internal Server Error

# Issue details

When adding a big website (i.e. https://norvig.com/big.txt ) to wallabag, I get a "Server error" notification with the exception `com.di72nn.stuff.wallabag.apiwrapper.exceptions.UnsuccessfulResponseException` and the text "HTTP response: 500 Internal Server Error"

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

### Actual behaviour
- Tell us what happens

See the issue details.

### Expected behaviour
- Tell us what should happen

There should be a notification that clarifies what went wrong, not an exception. Maybe there shouldn't be a notification at all.

# Steps to reproduce the issue
1. Add https://norvig.com/big.txt to the wallabag server ( I used the share option in Opera on my phone to do this)
2. Start syncing on the phone
3. Get the exception
Note: After a random amount of retries, the article will be downloaded to the phone. However, the website gets downloaded without any content. I don't know if this is a problem with the content fetcher, as the item doesn't have any text on the server too.

# Environment details
- **wallabag app version**: 2.0.2-DEBUG (106)
- **wallabag app installation source** (e.g. Gplay, F-Droid, manual): GitHub Master
- **Android OS version**: 7.1.2
- **Android ROM** (e.g. stock, LineageOS, SlimRom,…): LineageOS
- **Android hardware**: Moto G2 (thea)
- **wallabag server version**: 2.2.3

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

## Web server
Please paste relevant web server logs (e.g. from nginx, Apache, …) here:
Nothing was written in var/logs/prod.log.
**NOTE:** Be super sure to remove sensitive data like passwords, note that everybody can look here!

# 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)
Yup, works super fine when it is set up :) Never had any critical issues.