add comments to exported funcs and fix EndpointErr unit test
diff --git a/openstack/endpoint_location.go b/openstack/endpoint_location.go
index 3e64911..1184b34 100644
--- a/openstack/endpoint_location.go
+++ b/openstack/endpoint_location.go
@@ -47,7 +47,7 @@
 	}
 
 	// Report an error if there were no matching endpoints.
-	return "", gophercloud.ErrEndpointNotFound
+	return "", &gophercloud.ErrEndpointNotFound{}
 }
 
 // V3EndpointURL discovers the endpoint URL for a specific service from a Catalog acquired
@@ -87,5 +87,5 @@
 	}
 
 	// Report an error if there were no matching endpoints.
-	return "", gophercloud.ErrEndpointNotFound
+	return "", &gophercloud.ErrEndpointNotFound{}
 }