Propagate json error object to the caller

When plugin errors out, it prints out a JSON object to stdout
describing the failure. This object needs to be propagated out
through the plugins and to the container runtime. This change
also adds Print method to both the result and error structs
for easy serialization to stdout.

plugin/types.go

@@ -17,6 +17,7 @@ package plugin
 import (
 	"encoding/json"
 	"net"
+	"os"
 
 	"github.com/appc/cni/pkg/ip"
 )
@@ -36,6 +37,10 @@ type Result struct {
 	IP6 *IPConfig `json:"ip6,omitempty"`
 }
 
+func (r *Result) Print() error {
+	return prettyPrint(r)
+}
+
 // IPConfig contains values necessary to configure an interface
 type IPConfig struct {
 	IP      net.IPNet
@@ -54,6 +59,14 @@ type Error struct {
 	Details string `json:"details,omitempty"`
 }
 
+func (e *Error) Error() string {
+	return e.Msg
+}
+
+func (e *Error) Print() error {
+	return prettyPrint(e)
+}
+
 // net.IPNet is not JSON (un)marshallable so this duality is needed
 // for our custom ip.IPNet type
 
@@ -110,3 +123,12 @@ func (r *Route) MarshalJSON() ([]byte, error) {
 
 	return json.Marshal(rt)
 }
+
+func prettyPrint(obj interface{}) error {
+	data, err := json.MarshalIndent(obj, "", "    ")
+	if err != nil {
+		return err
+	}
+	_, err = os.Stdout.Write(data)
+	return err
+}