When using completion, the number of the word for which the shell
requests completion is provided in the environment variable
`NIX_GET_COMPLETIONS`. When the number smaller than 1 is or larger
than the number of arguments nix coredumps as a assert is violated.
This change removes the assert and instead throws an exception informing
the user that their autocomplete is most likely misconfigured.
Change-Id: I821719e470e576b6f63c06beb097338b53d183e0
git@71rd.net
20d50b049d