(This request is very similar to #728, but with source_up)

Right now, source_up searches upwards (using find_up) until it finds a file. If it doesn't, it exits 1, and if you've got strict_env turned on, loading the .envrc file immediately stops.

Is this intentional? The docstring on source_up sort of makes it sound like maybe it was intended to happily ignore missing files?

Loads another ".envrc" if found with the find_up command.

If it is intentional that source_up exits nonzero when there's no ancestor file, how would you feel about a PR adding a new source_up_if_exists method?