Remove the embedded error and satisify the builtin error interface

This also adds tests to verify that the created error string follows
the expected format of "code: message" and tests to verify that Error
can be used as an error.  Additionally, the idiom

var _, _ error = Error{}, &Error{}

was added so the build will fail if the builtin error interface is
never satisified in the future.

jsonapi.go

@@ -6,7 +6,6 @@ package btcjson
 
 import (
 	"encoding/json"
-	"errors"
 	"fmt"
 )
 
@@ -158,7 +157,15 @@ type ListUnSpentResult struct {
 type Error struct {
 	Code    int    `json:"code,omitempty"`
 	Message string `json:"message,omitempty"`
-	Error   error  `json:"-"`
+}
+
+// Guarantee Error satisifies the builtin error interface
+var _, _ error = Error{}, &Error{}
+
+// Error returns a string describing the btcjson error.  This
+// satisifies the builtin error interface.
+func (e Error) Error() string {
+	return fmt.Sprintf("%d: %s", e.Code, e.Message)
 }
 
 // jsonWithArgs takes a command, an id,  and an interface which contains an
@@ -723,7 +730,6 @@ func ReadResultCmd(cmd string, message []byte) (Reply, error) {
 	var jsonErr Error
 	var id interface{}
 	err = json.Unmarshal(objmap["error"], &jsonErr)
-	jsonErr.Error = errors.New(string(jsonErr.Code) + ": " + jsonErr.Message)
 	if err != nil {
 		err = fmt.Errorf("Error unmarshalling json reply: %v", err)
 		return result, err

jsonapi_test.go

@@ -7,6 +7,7 @@ package btcjson_test
 import (
 	"bytes"
 	"encoding/json"
+	"fmt"
 	"github.com/conformal/btcjson"
 	"io"
 	"io/ioutil"
@@ -381,3 +382,51 @@ func TestReadResultCmd(t *testing.T) {
 
 	return
 }
+
+// TestErrorInterface tests that the Error type satisifies the builtin
+// error interface and tests that the error string is created in the form
+// "code: message".
+func TestErrorInterface(t *testing.T) {
+	codes := []int{
+		-1,
+		0,
+		1,
+	}
+	messages := []string{
+		"parse error",
+		"error getting field",
+		"method not found",
+	}
+
+	// Create an Error and check that both Error and *Error can be used
+	// as an error.
+	var jsonError btcjson.Error
+	var iface interface{} = jsonError
+	var ifacep interface{} = &jsonError
+	if _, ok := iface.(error); !ok {
+		t.Error("cannot type assert Error as error")
+		return
+	}
+	if _, ok := ifacep.(error); !ok {
+		t.Error("cannot type assert *Error as error")
+		return
+	}
+
+	// Verify jsonError is converted to the expected string using a few
+	// combinations of codes and messages.
+	for _, code := range codes {
+		for _, message := range messages {
+			// Create Error
+			jsonError := btcjson.Error{
+				Code:    code,
+				Message: message,
+			}
+
+			exp := fmt.Sprintf("%d: %s", jsonError.Code, jsonError.Message)
+			res := fmt.Sprintf("%v", jsonError)
+			if exp != res {
+				t.Errorf("error string '%s' differs from expected '%v'", res, exp)
+			}
+		}
+	}
+}