Error message payload should be an array as defined in graphql-ws protocol

[jord1e]

Fixes #200

This is my first ever Spring contribution, please let me know if anything should be changed.

Is there a replacement for Collections#singletonList in Spring?

Documentation describing expected behaviour:

• https://github.com/enisdenjo/graphql-ws/blob/master/docs/interfaces/common.ErrorMessage.md

Screenshots

In this example I modified

spring-graphql/samples/webflux-websocket/src/main/java/io/spring/sample/graphql/DataRepository.java

Line 52 in 4d99d2a

return Flux.just("Hi", "Bonjour", "Hola", "Ciao", "Zdravo").map(s -> s + " " + name);

to the following:

return Flux.just("Hi", "Bonjour", "Hola", "Ciao", "Zdravo").map(s -> {
    if (s.equalsIgnoreCase("Zdravo")) {
        throw new RuntimeException("Test Exception");
    }
    return s + " " + name;
});

Before:

After:

Normal (without errors):

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[jvalkeal]

I tried this on top of my #204 and errors are now coming through fine.

Previously if you did:

subscription {
  xxxx
}

GraphiQL showed:

{
  "errors": {
    "message": "Invalid message",
    "stack": "Error: Invalid message\n    at f (http://localhost:8080/graphiql/main.js:13:3297)\n    at WebSocket.o.onmessage (http://localhost:8080/graphiql/main.js:13:7311)"
  }
}

Now it correctly shows error from ws payload:

{
  "errors": {
    "message": "Execution failed: [ValidationError{validationErrorType=FieldUndefined, queryPath=[xxxx], message=Validation error of type FieldUndefined: Field 'xxxx' in type 'Subscription' is undefined @ 'xxxx', locations=[SourceLocation{line=33, column=3}], description='Field 'xxxx' in type 'Subscription' is undefined'}]",
    "stack": "Error: Execution failed: [ValidationError{validationErrorType=FieldUndefined, queryPath=[xxxx], message=Validation error of type FieldUndefined: Field 'xxxx' in type 'Subscription' is undefined @ 'xxxx', locations=[SourceLocation{line=33, column=3}], description='Field 'xxxx' in type 'Subscription' is undefined'}]\n    at Object.error (http://localhost:8080/graphiql/main.js:37:202952)\n    at Object.f67f4083-ca61-4bd2-a976-600e678d9985 (http://localhost:8080/graphiql/main.js:13:9048)\n    at emit (http://localhost:8080/graphiql/main.js:13:5564)\n    at Object.emit (http://localhost:8080/graphiql/main.js:13:6112)\n    at WebSocket.o.onmessage (http://localhost:8080/graphiql/main.js:13:7323)"
  }
}