Adding helpful jextract error messages

[Ty Young]

Hi,


Currently the new jextract does not give any error messages as to why an 
attempted extraction has failed and will instead just print the 
key-switch help text over and over again.


Could this be improved? I'm trying to use a modified old jextract 
extraction command to dump struct layout information:


jextract -t org.nvidia.nvxctrl -lXNVCtrl  /usr/include/X11/Xmd.h 
/usr/include/X11/Xlibint.h /usr/include/NVCtrl/nv_control.h 
/usr/include/NVCtrl/NVCtrlLib.h --source


...but it fails. It seems like there is an issue where unless literal 
quotes(for example, "/usr/include/X11/Xmd.h") aren't used then it fails 
and multiple header file dumping isn't yet supported.