ibmcloud pi service-list should show Unauthorized error message if appropriate rather than an empty result

When I used the API key for a Service ID we had created, no workspaces were listed:

$ ibmcloud pi service-list

Listing services under account [REDACTED] as user ...

No instances found in the current account.

This is wrong in two ways:

1. There is a workspace there which I can see when logged in to the web UI

2. There is no error message indicating lack of authorization or permissions to view the list of workspaces for this account

I discovered that the Access Group for this Service ID was missing, (apparently deleted by another engineer on this project while fixing an unrelated issue) so I created it again in the web UI. Once I had created the Access Group, the same command showed the desired output:

$ ibmcloud pi service-list

Listing services under account Splunk CI-CD Production as user ...

ID Name

crn:v1:bluemix:public:power-iaas:[REDACTED] [REDACTED]

Please change the `ibmcloud pi service-list` command to indicate an error message like `ERROR 403 Forbidden: You don't have permissions to access the list of service instances associated with this account` rather than merely listing an empty result.