It seems that a `Glib::ConvertError` is thrown, and then the
function that catches the error causes another ConvertError in
`err.what()` (which presumably include the filename that
causes the error).
relevant part of the backtrace (macOS Crashdump):
```
Glib::ConvertError::throw_func(_GError*) + 56
Glib::Error::throw_exception(_GError*) + 292
Glib::operator<<(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, Glib::ustring const&) + 131
StringPrivate::Composition& StringPrivate::Composition::arg<Glib::ustring>(Glib::ustring const&) + 25
std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > string_compose<Glib::ustring> (...) + 81
PBD::run_functor_for_paths(...) + 1313
```
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