`repl-overlays` that fail to evaluate should error.
@args --repl-overlays
@args ${PWD}/extra_data/repl-overlay-fail.nix
@should-start false
  error:
         … while calling the 'foldl'' builtin
           at «string»:5:13:
              4| functions:
              5| let final = builtins.foldl'
               |             ^
              6|               (prev: function: prev // (function info final prev))

         … in the right operand of the update (//) operator
           at «string»:6:37:
              5| let final = builtins.foldl'
              6|               (prev: function: prev // (function info final prev))
               |                                     ^
              7|               initial

         (stack trace truncated; use '--show-trace' to show the full trace)

         error: evaluation aborted with the following error message: 'uh oh!'