"WorkFlow show" of a workflow that does not exist will not report an error

[aquarapid]

On current master (d6c9ddf), doing a WorkFlow show command of a random workflow name will "succeed" and show some output, even though that workflow does not exist.

E.g.:

• Setup local examples:

$ examples/local/
$ sh -x 101_initial_cluster.sh
.
.

• List a workflow that does not exist in the commerce keyspace, e.g. foo123:

$ vtctlclient -server localhost:15999 WorkFlow commerce.foo123 show 
{
      "Workflow": "foo123",
      "SourceLocation": {
              "Keyspace": "",
              "Shards": []
      },
      "TargetLocation": {
              "Keyspace": "commerce",
              "Shards": [
                      "0"
              ]
      },
      "MaxVReplicationLag": 0,
      "ShardStatuses": {
              "0/zone1-0000000100": {
                      "MasterReplicationStatuses": null,
                      "TabletControls": null,
                      "MasterIsServing": true
              }
      }
}

i.e. the workflow is shown as empty; which isn't really true, it doesn't exist at all. This is bad UX, since users might get confused if they fat-finger the workflow name. We should return an error instead.

[aquarapid]

This was fixed by #7008 ; verified against 9.0 RC1