gnosis / dex-services / 4513

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Handle internal server errors (#1240)

Fixes #1141

This PR implements explicit handling of the `InternalError` warp rejection.

### Test Plan

Unfortunately this error is not so easy to trigger, so apply the following patch:
```diff
diff --git a/price-estimator/src/filter.rs b/price-estimator/src/filter.rs
index 1a2f551..3a1445a 100644
--- a/price-estimator/src/filter.rs
+++ b/price-estimator/src/filter.rs
@@ -174,7 +174,8 @@ async fn get_markets(
     orderbook: Arc<Orderbook>,
 ) -> Result<impl Reply, Rejection> {
     if !query.atoms {
-        return Err(warp::reject());
+        //return Err(warp::reject());
+        return Err(error::internal_server_rejection(anyhow::anyhow!("test")));
     }
     let transitive_orderbook = orderbook
         .pricegraph(query.batch_id, PricegraphKind::Raw)
```

Run the price estimator, and trigger the error:
```
$ curl -s 'http://localhost:8080/api/v1/markets/1-7?atoms=false' | jq
{
  "message": "internal server error"
}
```

Also, notice how the error information appears in the logs:
```
$ cargo run -p price-estimator
...
[2020-08-03T13:47:11Z WARN  price_estimator::filter] internal server error: test
...
```

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