Targets to generate warning/error/info messages from userland

[raphaelvigee]

Before execution, heph would create a file, pass its absolute path in the target as an env var HEPH_ERROR

Userland targets could be writing into that file which would be watched by heph during execution, any line written to it would be converted to a log.Warn/Error/Info

echo "Something is failing" >> $HEPH_ERROR
echo "Watch out for crocodiles!" >> $HEPH_WARN

See https://gist.github.com/matishsiao/fc1601a3a3f37c70d91ab3b1ed8485c4