chore: Disable fetching new emails after mailbox error (#4176)

- Disabled email fetch job if credentials for the channel isn't working
- notify customers when the email channel isn't working

fixes: https://github.com/chatwoot/chatwoot/issues/4174

app/mailboxes/mailbox_helper.rb

@@ -5,7 +5,7 @@ module MailboxHelper
     @message = @conversation.messages.create(
       account_id: @conversation.account_id,
       sender: @conversation.contact,
-      content: mail_content,
+      content: mail_content&.truncate(150_000),
       inbox_id: @conversation.inbox_id,
       message_type: 'incoming',
       content_type: 'incoming_email',