setup.hs: substitute uses only valid bash names

bash variable names may only contain alphanumeric ASCII-symbols and _,
and must not start with a number. Nix expression attribute names however
might contain nearly every character (in particular spaces and dashes).

Previously, a substitution that was not a valid bash name would be
expanded to an empty string. This commit introduce a check that throws
a (hopefully) helpful error when a wrong name is used in a substitution.