Campaigns don't surface sender authentication errors

[almereyda]

When running a campaign with a sender whose password had been changed, there are several error message of the following kind per recipient in the application log:

[warning] Failed sending email to [email protected] for campaign nmc_G4LyxgjB: {:no_more_hosts, {:permanent_failure, ~c"mail.example.com", :auth_failed}}

These errors are not surfaced to the user sending the campaign, leaving them with no clue why nothing happened.

It could be nice if intermediate, incomplete states of campaigns would also be available to the user initiating them.