Suggestion: nullmailer-inject could have a more helpful error message when it cannot run nullmailer-queue

[hacklschorsch]

Hi & thanks for Nullmailer!

I was debugging the mail sending of an unrelated software I configured to use Nullmailer today which only gave me this not very helpful log entry:

nullmailer-inject: nullmailer-queue failed: 1

It took me longer than I like to admit to figure out this was because the caller - a service running under systemd - was configured to not be allowed gaining privileges with the systemd directive NoNewPrivileges=true.
This breaks calling nullmalier-queue because of its using setuid.

Since Nullmailer has lots of very useful error messages when something goes wrong, I´d like to suggest the not being able to run nullmailer-queue because of this could have a more helpful/verbose error message as well.