Better error for empty repos option

[rizo]

When repos = [] onix will fail with:

onix: internal error, uncaught exception:
      Failure("Invalid repo format: ")
      Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
      Called from Stdlib__List.map in file "list.ml", line 92, characters 20-23
      Called from Onix_core__Nix_utils.resolve_repos in file "src/onix_core/Nix_utils.ml", line 198, characters 27-51
      Called from Onix_core__Solver.solve in file "src/onix_core/Solver.ml", line 46, characters 4-43
      Called from Dune__exe__Main.Lock.run in file "src/Main.ml", line 224, characters 6-119
      Called from Cmdliner_term.app.(fun) in file "cmdliner_term.ml", line 24, characters 19-24
      Called from Cmdliner_eval.run_parser in file "cmdliner_eval.ml", line 34, characters 37-44