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,
+	}
+}