add core/error/util files

pkg/errs/system.go

@@ -0,0 +1,9 @@
+package errs
+
+import "net/http"
+
+var (
+	ErrSystemError      = NewSystemError(SYSTEM_SUBCATEGORY_DEFAULT, 0, http.StatusInternalServerError, "system error")
+	ErrApiNotFound      = NewSystemError(SYSTEM_SUBCATEGORY_DEFAULT, 1, http.StatusNotFound, "api not found")
+	ErrMethodNotAllowed = NewSystemError(SYSTEM_SUBCATEGORY_DEFAULT, 2, http.StatusMethodNotAllowed, "method not allowed")
+)