fixup! disruptors: error out if there are no targets

extract group printing logic out of String to a different function

pkg/disruptors/pod.go

@@ -74,24 +74,8 @@ func (p PodSelector) String() string {
 		str = "all pods"
 	} else {
 		str = "pods "
-		if len(p.Select.Labels) != 0 {
-			str += "including("
-			for k, v := range p.Select.Labels {
-				str += fmt.Sprintf("%s=%s, ", k, v)
-			}
-			str = strings.TrimSuffix(str, ", ")
-			str += "), "
-		}
-
-		if len(p.Exclude.Labels) != 0 {
-			str += "excluding("
-			for k, v := range p.Exclude.Labels {
-				str += fmt.Sprintf("%s=%s, ", k, v)
-			}
-			str = strings.TrimSuffix(str, ", ")
-			str += "), "
-		}
-
+		str += p.groupLabels("including", p.Select.Labels)
+		str += p.groupLabels("excluding", p.Exclude.Labels)
 		str = strings.TrimSuffix(str, ", ")
 	}
 
@@ -100,6 +84,24 @@ func (p PodSelector) String() string {
 	return str
 }
 
+// groupLabels returns a group of labels as a string, giving that group a name. The returned string has the form of:
+// `groupName(foo=bar, boo=baz), `, including the trailing space and comma.
+// An empty group of labels produces an empty string.
+func (PodSelector) groupLabels(groupName string, labels map[string]string) string {
+	if len(labels) == 0 {
+		return ""
+	}
+
+	group := groupName + "("
+	for k, v := range labels {
+		group += fmt.Sprintf("%s=%s, ", k, v)
+	}
+	group = strings.TrimSuffix(group, ", ")
+	group += "), "
+
+	return group
+}
+
 // NewPodDisruptor creates a new instance of a PodDisruptor that acts on the pods
 // that match the given PodSelector
 func NewPodDisruptor(