Support reporting errors on stderr

[TheBigRoomXXL]

Hello,

This change allows to report errors on stderr. It only adds 4 lines and it's behind a flag reportonoutage so that it doesn't change the default behavior.

This is useful when running tinystatus with cron as you can just add a MAILTO tag to receive alerts by mail without additional setup.

Thank you for this little script! It's really nice and makes my life simpler!