Only return 500 on /send if a database error occurs (#1503)

federationapi/routing/send.go

@@ -16,6 +16,7 @@ package routing
 
 import (
 	"context"
+	"database/sql"
 	"encoding/json"
 	"fmt"
 	"net/http"
@@ -234,18 +235,11 @@ func (t *txnReq) processTransaction(ctx context.Context) (*gomatrixserverlib.Res
 // we should stop processing the transaction, and returns false if it
 // is just some less serious error about a specific event.
 func isProcessingErrorFatal(err error) bool {
-	switch err.(type) {
-	case roomNotFoundError:
-	case *gomatrixserverlib.NotAllowed:
-	case missingPrevEventsError:
-	default:
 	switch err {
-		case context.Canceled:
-		case context.DeadlineExceeded:
-		default:
+	case sql.ErrConnDone:
+	case sql.ErrTxDone:
 		return true
 	}
-	}
 	return false
 }