istioctl experimental wait has undecipherable message if value isn't set

[ericvn]

Bug Description

I get this message trying to use wait when Pilot Version tracking is disabled (the default).

istioctl experimental wait gateway httpbin-gateway
Error: invalid character 'P' looking for beginning of value

I added some logic to dump the response that since it seems to be a unmarshalling type error.

The response was:

Pilot Version tracking is disabled.  Please set the PILOT_ENABLE_CONFIG_DISTRIBUTION_TRACKING environment variable to true to enable.

It would be good to check for the existence of this value and return a better message from the istioctl experimental wait.

Version

> istioctl version                                                                                                        
no running Istio pods in "istio-system"
1.16.0

Although I'm not sure which release made setting the value a requirement. I assume this is broken back that far.

Additional Information

No response