Capture and return errors during watches on consul

Before this change consul errors were not returned correctly
during watches. Resolved the issue by returning the initial
watch index and error and avoid the nil pointer dereference.

Fixes #322

backends/consul/client.go

@@ -83,6 +83,10 @@ func (c *ConsulClient) WatchPrefix(prefix string, waitIndex uint64, stopChan cha
 			WaitIndex: waitIndex,
 		}
 		_, meta, err := c.client.List(prefix, &opts)
+		if err != nil {
+			respChan <- watchResponse{waitIndex, err}
+			return
+		}
 		respChan <- watchResponse{meta.LastIndex, err}
 	}()
 	for {