[darc/maestro++] Build shouldn't silently finish if it's supposed to publish to a channel that doesn't have a publishing implementation set up.

-  Is this issue blocking (no)
-  Is this issue causing unreasonable pain (yes)

https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=3441625

It looks like this PR https://github.com/dotnet/toolset/pull/4223 got a maestro update with a bad build.