util: add new error checking code in vbuf helper

util: add new error checking code in vbuf helper

Check the return value of calls to u_upload_alloc() and
u_upload_data() and return early if needed.

Since we don't have a way to propagate errors all the way up to
Mesa through pipe_context::draw_vbo(), call debug_warn_once() so
the user might have some clue about OOM errors.

Note: This is a candidate for the 9.0 branch.