`seq`: propogate errors to `uumain`

In reference to https://github.com/uutils/coreutils/pull/3112#discussion_r804508653 and the comment afterwards, it would be nice if we could propagate InvalidSpec errors from that PR through seq.rs's uu_main. Currently, we do the following in seq.rs in two places:

if let Err(x) = Memo::run_all(f, &[s]) {
    show!(x);
    exit(1);
}

Propagating this error as opposed to the above proves to be non-trivial, however, because of this ErrorKind check - if we generalize print_seq()/print_seq_integers() to return a UResult as opposed to an io::Result then we lose the ability to handle BrokenPipes gracefully.

Start in src/uu/seq/src/seq.rs, especially uu_main, the two Memo::run_all calls, print_seq(), print_seq_integers(), and the ErrorKind check around lines 141–145. Trace how InvalidSpec is currently shown and exited, then preserve graceful BrokenPipe handling while propagating the error through uu_main. Done means InvalidSpec reaches uu_main without losing BrokenPipe behavior.