Multipass needs a top-level catch-all

[ricab]

See this. In general, things can throw, so in C++ there should be a top-level catch; or generically, a catch-all in the top-most function that calls anything that is not noexcept.

CLI and GUI clients seem to be missing this. #618 is related.

For a practical example, just change the permissions/format of one of the files we need.