Make "Missing endpoint..." error more informative

Now it's "Missing endpoint, or insufficient privileges to access endpoint; criteria = %# v"

Having the `criteria` shown makes it clear why it's failing.

Take this example:

```
Missing endpoint, or insufficient privileges to access endpoint; criteria = gophercloud.ApiCriteria{Name:"nova", Type:"", Region:"RegionOne", VersionId:"", UrlChoice: 0, IgnoreEnvVars:false}
```

By looking at the value of `criteria`, we can see that it was asking for an API with the name `"nova"`. And apparently it didn't find one in the `ServiceCatalog`.