Implement Vuls's own error code (#812)

* add error pkg

* fix fmt format

* fix NewError -> New

* fix err msg format

errof/errof.go

@@ -0,0 +1,27 @@
+package errof
+
+// ErrorCode is vuls error code
+type ErrorCode string
+
+// Error is vuls error
+type Error struct {
+	Code    ErrorCode
+	Message string
+}
+
+func (e Error) Error() string {
+	return e.Message
+}
+
+var (
+	// ErrFailedToAccessGithubAPI is error of github alert's api access
+	ErrFailedToAccessGithubAPI ErrorCode = "ErrFailedToAccessGithubAPI"
+)
+
+// New :
+func New(code ErrorCode, msg string) Error {
+	return Error{
+		Code:    code,
+		Message: msg,
+	}
+}

github/github.go

@@ -26,11 +26,11 @@ import (
 	"time"
 
 	"github.com/future-architect/vuls/config"
+	"github.com/future-architect/vuls/errof"
 	"github.com/future-architect/vuls/models"
 	"github.com/future-architect/vuls/util"
 	"github.com/k0kubun/pp"
 	"golang.org/x/oauth2"
-	"golang.org/x/xerrors"
 )
 
 // FillGitHubSecurityAlerts access to owner/repo on GitHub and fetch scurity alerts of the repository via GitHub API v4 GraphQL and then set to the given ScanResult.
@@ -75,7 +75,10 @@ func FillGitHubSecurityAlerts(r *models.ScanResult, owner, repo, token string) (
 
 		util.Log.Debugf("%s", pp.Sprint(alerts))
 		if alerts.Data.Repository.URL == "" {
-			return 0, xerrors.Errorf("Failed to access to GitHub API. Response: %#v", alerts)
+			return 0, errof.New(
+				errof.ErrFailedToAccessGithubAPI,
+				fmt.Sprintf("Failed to access to GitHub API. Response: %#v", alerts),
+			)
 		}
 
 		for _, v := range alerts.Data.Repository.VulnerabilityAlerts.Edges {