Updater Constructor does not actually throw std::invalid_argument

[Diyou]

I want to create a wrapper of libappimageupdate for c# (#203).
While working with the code I found that the latter part of the constructor does not actually throw the invalid_argument exception when given an invalid path:
https://github.com/AppImage/AppImageUpdate/blob/2708f91c9ba86eefbc3f13e666cc3356ab9a21a9/src/updater.cpp#L208-L219

AppImageError is thrown in the initializer list of the constructor at:
https://github.com/AppImage/AppImageUpdate/blob/2708f91c9ba86eefbc3f13e666cc3356ab9a21a9/src/updater.cpp#L38

This confused me for a while because I was expecting an ArgumentException in c# and I needed to wrap for std::runtime_error to catch the ApplicationException.