Alternative fix for #427

[rogierschouten]

Issue #427 already has a PR #451 but this solution is a bit more flexible.

This fixes notably the issue that when the client forgets a mandatory field in a GraphQL mutation input,
the underlying graphql-js package does not throw; rather it returns a normal response with errors. This should result in status code 400 (bad request) because it is a client problem, but express-graphql makes it a 500.

This PR allows to specify separately what to do with normal responses that

• contain no data and no errors (typically 500)
• contain no data and errors (typically 400)
• contain both data and errors (typically 200)

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[jsonmaur]

@rogierschouten I was about to open a similar PR for this. It really needs to be fixed... issues with the data coming from clients should never return a 500 status code.

@IvanGoncharov Can we get this merged?

[jsonmaur]

After looking at other PR's, this idea of using Express middleware for error handling might be a better approach.