Synapse sends stack trace as error message over federation

This issue has been migrated from [#6582](https://github.com/matrix-org/synapse/issues/6582).

---

## Description

Synapse 1.7.1 sends stack tarce over fedaration as error message. It begins like this:

```
Dec 20 12:57:52 bubu synapse[125805]: 2019-12-20 12:57:52,138 - synapse.federation.sender.transaction_manager - 149 - WARNING - federation_transaction_transmission_loop-1606 - TX [] {1576842828228} Remote returned error for $15768430190XSQqk:bubu1.eu: {'error': 'FirstError[#0, [Failure instance: Traceback: : duplicate key value violates unique constraint "current_state_events_event_id_key"\nDETAIL: Key (event_id)=($1561762643543412lzqRA:matrix.org) already exists.
```

Following are python path installation details of the sending server. While these are not super secret, it's still some sort of information disclosure about the local installation that shouldn't be send to other servers.

### Version information

- **Homeserver**: bubu1.eu

If not matrix.org:

- **Version**: 1.7.2