ffi_type: tweak error handling
ffi_type: tweak error handling

Handle fix_external_name failure better.

GuillaumeGomez/gir,gtk-rs/gir,insanebits/gir,insanebits/gir,gkoz/gir,gkoz/gir,GuillaumeGomez/gir,GuillaumeGomez/gir,gtk-rs/gir,EPashkin/gir,gtk-rs/gir,EPashkin/gir