As far as I know,
nightly is a word of your choosing, so there’s no “nightly” feature, as such. You’re using the
schedule feature, which in your case runs at 11am every morning (based on East-coast US server time, I would guess).
I don’t think your selective notifications are possible out of the box, but you could look at the Circle API. You could scan that with a script and push a message to Slack if it finds a recent failed build. That could be run as another scheduled build process in Circle, in fact.
Actually, that may be over-complicating it. You could just run something inside the same build process to send you a Slack message on failure. The only minor issue here is detecting that the build has been done on the
nightly trigger rather than on push - you could use the time to detect that.