Align NetworkError and GraphQLErrors types

[korywka]

The problem:

Application-wide networkError error handler fails with error:

TS2322: Type 'Error | ServerParseError | ServerError | null' is not assignable to type 'Error | ServerParseError | ServerError | undefined'.   Type 'null' is not assignable to type 'Error | ServerParseError | ServerError | undefined'. 

Seems to be due to:

ApolloError networkError have Error | ServerParseError | ServerError | null type:

export declare class ApolloError extends Error {
    message: string;
    graphQLErrors: ReadonlyArray<GraphQLError>;
    networkError: Error | ServerParseError | ServerError | null;

But the same networkError in ErrorResponse have Error | ServerError | ServerParseError type:

export interface ErrorResponse {
    graphQLErrors?: ReadonlyArray<GraphQLError>;
    networkError?: Error | ServerError | ServerParseError;

The difference is in null. To keep backward compatibility as much as possible, I am adding null to Link ErrorResponse interface.

The next step may be to remove null and make it optional.

[apollo-cla]

@korywka: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[brainkim]

Thanks again @korywka!