Warning in log when a Slack channel import fails. (#4262)

Previously it just wrote to the summary, and as a debug level message in
the log, so if you had the log level set to warn, you might never find
out that importing a channel failed.

i18n/en.json

@@ -1532,8 +1532,8 @@
     "translation": "Failed to import: {{.DisplayName}}\r\n"
   },
   {
-    "id": "api.slackimport.slack_add_channels.import_failed.debug",
-    "translation": "Failed to import: %s"
+    "id": "api.slackimport.slack_add_channels.import_failed.warn",
+    "translation": "Slack Importer: Failed to import channel: %s"
   },
   {
     "id": "api.slackimport.slack_add_channels.merge",