Drop `details` support in cloud error ResponseError

[mstoykov]

Why?

Currently the parsing of error from the cloud api has some complicated code

https://github.com/grafana/k6/blob/b5a6febd56385326ea849bde25ba09ed6324c046/cloudapi/errors.go#L47-L62

Almost all of which is around parsing details which was teh old way the error was returned but has been deprecated and not needed for 5 years.

What?

We can probably just drop most of the code but some testing is needed.