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
rbenv --version
rbenv-exec
help
realpath
rbenv hooks
rbenv.d
rbenv-rehash
configure
rbenv init
rbenv versions --skip-aliases
assert_output_lines