Incorrect error message format for the graphql-ws protocol

[ReneeSlamka]

Hi, I've found a gap in support for the legacy graphql-ws protocol. The spec indicates that for error messages the payload should be a single "Error" object, but currently Mercurius only adheres to the graphql-transport-ws protocol error format by always returning an array of GraphQL errors.

I realize that the graphql-ws protocol is considered deprecated, but given the docs state both are supported I think this is worth addressing. Thanks!

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.