discovery: print out detailed cluster error

Conflicts:
	discovery/discovery.go

client/cluster_error.go

@@ -14,6 +14,8 @@
 
 package client
 
+import "fmt"
+
 type ClusterError struct {
 	Errors []error
 }
@@ -21,3 +23,11 @@ type ClusterError struct {
 func (ce *ClusterError) Error() string {
 	return ErrClusterUnavailable.Error()
 }
+
+func (ce *ClusterError) Detail() string {
+	s := ""
+	for i, e := range ce.Errors {
+		s += fmt.Sprintf("error #%d: %s\n", i, e)
+	}
+	return s
+}