If `foo` didn't exist and `RBENV_VERSION=system rbenv which foo` was called, the error message used to be misleading: rbenv: version `system' is not installed Instead, have the error message simply say that the command was not found. Fixes #770
$BASH_SOURCE
$0
rbenv --version
exec/which/whence
realpath
rbenv hooks
.bashrc
.bash_profile
rbenv prefix system
rbenv versions --skip-aliases
hash -r
rbenv init