exiv2 plugins segfault on non-image file

[Đoàn Trần Công Danh]

Hello,

On non-image file, exiv2 will call ExtractorIO::path() to get the path
to report the error. libextractor currently throw an exception, which
would be propagated to outer layer for error reporting.

However from exiv2 0.28.0, the function in question is noexcept, any
attempts to throw from that function will terminate the application
immediately.

I think it's better to make that function return empty string instead.

-- 
Danh