start: propagate init errors

package users can now act accordingly on startup errors, for example
by exiting with a non-zero code. this is useful when running a service,
notifying a supervising process about such failures.

although this is technically a breaking change, most pkg users should be
unaffected since Start and StartConf returned nothing before this commit.

basic/main.go

@@ -75,5 +75,7 @@ func main() {
 		return
 	}
 	r.storage = &postgresql.PostgresBackend{DatabaseURL: r.PostgresDatabase}
-	relayer.Start(&r)
+	if err := relayer.Start(&r); err != nil {
+		relayer.Log.Fatal().Err(err).Msg("server terminated")
+	}
 }