Function set_err_msg 

pub unsafe extern "C" fn set_err_msg(
    context: *mut c_context,
    err: err,
    msg: *const c_char,
) -> err

@brief Set an error message on a nix context.

This should be used when you want to throw an error from a PrimOp callback.

All other use is internal to the API.

@param context context to write the error message to, required unless C++ exceptions are supported @param err The error code to set and return @param msg The error message to set. This string is copied. @returns the error code set