jextract should report unsupported language and exit rather than generating partial nonworking code

jextract should report unsupported language and exit rather than generating partial code

jextract's -C option can be used to pass any clang compiler option. User may pass -C-xc++ or -C-xobjc and pass C++ or Objective C header. We don't support anything other than C as of now. It is better to report clear error message and exit.