GraphQL Resolvers Cannot Return Error Reference Types

[ThisaruGuruge]

Description:
GraphQL resolver functions (resource/remote) methods should be able to return custom error types. Currently, when you return a custom error, the compiler plugin throws an error.

Consider the following code:

import ballerina/graphql;

service on new graphql:Listener(9090) {
    resource function get greeting() string|graphql:Error {
        return error graphql:Error("Error");
    }
}

The above code does not compile and returns the following compilation error:

ERROR [main.bal:(9:5,14:6)] invalid GraphQL union type member 'string' found. Only distinct service types are allowed
ERROR [main.bal:(9:5,14:6)] invalid GraphQL union type member 'Error' found. Only distinct service types are allowed