Handle 413 (payload too large) somehow

```
POST /upload multipart/form-data:
   >> Matched: (upload) POST /upload
   >> `Form < Upload < '_ > >` data guard failed: Errors([Error { name: Some("upload"), value: None, kind: InvalidLength { min: None, max: Some(1048576) }, entity: Value }]).
   >> Outcome: Failure
   >> No 413 catcher registered. Using Rocket default.
   >> Response succeeded.
```