Return the original upstream response body when the the list request
returns an error. In general, the provider is returning useful and
actionable error messages for the user, the previous code hid this in
the logs and this is easy for user to overlook.

Additionally, remove an early return from error case after fetching
metrics. This looked like a bug and could result in empty api responses
if there was a prometheus error.

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>