Fallible one-shot systems silently swallow errors

[alice-i-cecile]

I see it got merged, but I don't think that PR quite works the way we want. It lets you use any output type with commands.run_system_cached, but it just drops the output with ;. So if you pass a fallible system and it fails, it will silently drop the Result::Err, and not pass it to the error handler.

From @chescock about #19678.