Make valid GraphQL error reply when Keystone backend is initializing

[MurzNN]

When the Keystone backend is offline (server is stopped), Admin UI shows the error:

NetworkError when attempting to fetch resource.

That is ok, but when Keystone is initializing (that can spend long time), Admin UI shows the error:

JSON.parse: unexpected character at line 1 column 1 of the JSON data

because on /api/graphql it receives the HTML with "Keystone is loading..." instead of JSON-formatted reply.

Maybe on initializing stage we can replace the HTML reply on GraphQL endpoint to some valid GraphQL reply with the same error text "Keystone is loading..."?

[bladey]

Thank you for the suggestion @MurzNN.