Remove ErrorResponse type

[displague]

Tthe ErrorResponse type copies the http.Response unnecessarily since all packngo API methods return both an error and the http.Response.

A more fitting error type would be one that simplifies the single and multiple errors returned from the Equinix Metal API. This could be done with a packngo.Error type (effectively ErrorResponse, without embedding *http.Response)

One missed advantage of the ErrorResponse type is that http.Response was embedded instead of packngo.Response (which offers insight into Equinix Metal API rate-limits).

In retrospect, I think it may have been a better pattern to return (for example) Devices.List() ([]Device, *packngo.Response, error) (with the error being an packngo.Error in most cases).

[displague]

Related conversations in the Terraform provider:

• Fix ErrorResponse interface to satisfy TF SDK equinix/terraform-provider-metal#197 (comment)
• fix 403 and 404 detection equinix/terraform-provider-metal#192 (comment)

[displague]

In Terraform, an ErrorResponse type was created mimicking the packngo.ErrorResponse type. This has created difficulty in handling errors (as expressed in the comments listed above).