Feature request: -n option to check exit code

[fragglet]

All the major BSDs support a -n prefix before the command that only mails if the command fails. From the OpenBSD documentation:

     -n command
             No mail is sent after a successful run.  The execution output
             will only be mailed if the command exits with a non-zero exit
             code.  The -n option is an attempt to cure potentially copious
             volumes of mail coming from cron(8).

For some reason this doesn't seem to have made its way to Vixie Cron and its descendants, including cronie.

It seems like a common enough question on Stack Overflow, and people have all kinds of elaborate workarounds in the replies:

• https://serverfault.com/questions/226074/cron-only-get-errors-in-emails
• https://manpages.ubuntu.com/manpages/xenial/en/man1/cronic.1.html (the author of Cronic has an entire rant page about it

TO CLARIFY: the desired behavior here is:

• If the command exits with success (exit code 0), no email is sent
• If the command exits with failure (exit code non-zero), an email is sent containing stdout and stderr

NOT ALTERNATIVE SOLUTIONS:

• cmd >/dev/null 2>/dev/null: This blindly suppresses all failures
• cmd >/dev/null: Better because at least you get error output, but if the job fails you lose potentially useful information that was in stdout. Exit code is not checked
• cmd || echo job failed: Requires modification of cmd to suppress normal output, which may be difficult or impossible, and potentially lose valuable contextual information as to why the command failed

[t8m]

This was already implemented by #121 and is present in 1.7.0 release.