Make shared library error message helpful

[abergmeier]

I get an error:

thread 'main' panicked at 'couldn't write to "./src/lib.rs": Custom { kind: Other, error: "No shared library found, but functions were found" }'

@sdroege explained that it means:

your gir file is incomplete. it has no library entry

So maybe the error could be improved to:

No shared library found, but functions were found.
namespace {} in file {} has empty or missing shared_library entry.