Fix failure to import same Slack archive to 2 teams. (#4650)

This fixes the issue where you can't import 2 Slack exports with any
users who's emails appear in both exports onto different teams of the
same server.

Fixes #4126

i18n/en.json

@@ -1643,6 +1643,10 @@
     "id": "api.slackimport.slack_add_users.merge_existing",
     "translation": "Merged user with existing account: {{.Email}}, {{.Username}}\r\n"
   },
+  {
+    "id": "api.slackimport.slack_add_users.merge_existing_failed",
+    "translation": "Tried to merge user with existing account: {{.Email}}, {{.Username}}, but failed to add the user to this team.\r\n"
+  },
   {
     "id": "api.slackimport.slack_add_users.unable_import",
     "translation": "Unable to import user: {{.Username}}\r\n"