graphqlbackend: Do not return error on failing to resolve submodule
Created by: keegancsmith
This was a regression introduced in 86bf1b7f654519512d93895c321326b7ef31d89b. It lead to any repository containing unresolvable submodules to fail to load in Sourcegraph.
Test plan: visited https://sourcegraph.com/github.com/markus-wa/demoinfocs-golang on my local instance and it loaded.
Fixes https://github.com/sourcegraph/sourcegraph/issues/2947