carthage.sh and sh confusion

@kdienes noticed that it's fairly easy to get carthage.sh and sh confused:

from carthage import * makes sh be carthage.sh
carthage.sh defaults to background, and if the command fails, throws away the error unless you await
carthage.sh does not complain if youever await To fix:
Do not export carthage.sh in carthage's __all__
Throw a warning if a carthage.sh with _bg_exc=False is never awaited.

Hadron / carthage