Nested "on" blocks should raise or warn

From https://github.com/capistrano/sshkit/issues/348#issuecomment-220160838

> AFAIK nested on blocks were never considered in the design. The fact that it works at all is really just coincidence, due to how the SSHKit DSL is mixed into all objects. IMO it should raise an exception. You can open an SSHKit issue.

My personal opinion is that we should actually support it (take intersection of all role sets in the call stack), but until then, we should raise or warn when we find a nested `on` blocks.