Give better error if namespaces not enabled

[lkysow]

If the user is running Consul OSS by accident it would be nice to give them a better error message than:

2020-11-19T16:12:17.514Z [INFO]  Bootstrap token is provided so skipping Consul server ACL bootstrapping
2020-11-19T16:12:18.416Z [INFO]  Success: calling /agent/self to get datacenter
2020-11-19T16:12:18.416Z [INFO]  Current datacenter: datacenter=gcp-us-central-1
2020-11-19T16:12:18.610Z [INFO]  Policy "cross-namespace-policy" already exists, updating
2020-11-19T16:12:18.710Z [INFO]  Success: creating cross-namespace-policy policy
2020-11-19T16:12:18.711Z [ERROR] Error updating the default namespace to include the cross namespace policy: err="Unexpected response code: 404 ()"

How I've tested this PR: I didn't test it, I looked at the logs from an internal issue.

How I expect reviewers to test this PR: Code only. But possible to try and stand up a cluster with ACLs enabled, consul namespaces enabled and OSS binary.

Checklist:
No tests or changelog added because we don't test log messages and this isn't big enough for a changelog entry I don't think?

[kschoche]

looks great!