Fail if cabal update fails

[domenkozar]

Example:

$ touch ~/.cabal/packages/hackage.haskell.org/hackage-security-lock
$ ~/.local/bin/stack2nix .                                                                 
Ensuring git version is >= 2 ...                               
Ensuring cabal version is >= 2 ...
stack2nix: Failed with exit code 1...                                       
"/home/ielectric/.cabal/packages/hackage.haskell.org/hackage-security-lock: createDirectory: already e
xists (File exists)"                                                       
                                                                                   
CallStack (from HasCallStack):                                             
  error, called at src/Stack2nix/External/Util.hs:21:3 in stack2nix-0.1.3.1-HwgtPF9ZM2W8vYBIr0JOPS:Sta
ck2nix.External.Util                                     
                                      

fixes #92