webui cert manager: Add specific error message for importing a cert twice.

chrome/browser/ui/webui/options2/certificate_manager_handler2.cc

Index: chrome/browser/ui/webui/options2/certificate_manager_handler2.cc
diff --git a/chrome/browser/ui/webui/options2/certificate_manager_handler2.cc b/chrome/browser/ui/webui/options2/certificate_manager_handler2.cc
index e8add4777844034d551b3c14dd6caab67880a49d..e288814bd70a538ad8e119d957ad488675170c04 100644
--- a/chrome/browser/ui/webui/options2/certificate_manager_handler2.cc
+++ b/chrome/browser/ui/webui/options2/certificate_manager_handler2.cc
@@ -134,6 +134,9 @@ std::string NetErrorToString(int net_error) {
     // TODO(mattm): handle more cases.
     case net::ERR_IMPORT_CA_CERT_NOT_CA:
       return l10n_util::GetStringUTF8(IDS_CERT_MANAGER_ERROR_NOT_CA);
+    case net::ERR_IMPORT_CERT_ALREADY_EXISTS:
+      return l10n_util::GetStringUTF8(
+          IDS_CERT_MANAGER_ERROR_CERT_ALREADY_EXISTS);
     default:
       return l10n_util::GetStringUTF8(IDS_CERT_MANAGER_UNKNOWN_ERROR);
   }