Feature/handle errors in handler

[ekampf]

Added a _handle_graphql_errors(errors) hook to GraphQLHandle allowing processing errors after graphql finished execution.

The use case for this is that sometimes we want to turn certain GraphQL errors into http errors. Forexample if we're doing authentication login in the resolvers and want to convert authentication errors to 401 status response.

[coveralls]

Coverage increased (+0.07%) to 91.758% when pulling bf84fab1f0a3f9096473ad3630462b20b75d469a on feature/handle_errors_in_handler into 73642ce0ce83fe59e8a2acff20eb84fe4015e54a on master.