update with correction

fixed issues

Use hts_tpool_next_result_wait() in threadfn_orderedwrite() so it
blocks properly when it has nothing to do.  Remove lock and
conditional that are no longer needed.  Use a sentinel job to
tell threadfn_orderedwrite() that there is no more data and it can
terminate.

Improve error handling.  Add a data::result field so that
threadfn_orderedwrite() can report back to the main thread if
it encountered a problem.  Ensure everything is cleaned up
correctly if either the main thread or threadfn_orderedwrite()
fail, and in the right order.  Set the return value to EXIT_FAILURE
if either sam_close() or threadfn_orderedwrite() report failure.

Ensure error messages are written to stderr.

The "..." to indicate clipped out code is useful as it removes a lot
of the unnecessary boiler plate, especially error checking, and allows
us to drill down to the essense of what we're trying to teach.
However without indentation it's not clear what bits are being
removed.

Also there are times when the code removed is an error handler plus
a subsequent else clause, which gives the false impression with the
indentation that the if-clause has been negated.  Eg:

    ...
    //select required field alone, this is useful for CRAM alone
    if (hts_set_opt(infile, CRAM_OPT_REQUIRED_FIELDS, SAM_FLAG) < 0) {
    ...
       //read header
       in_samhdr = sam_hdr_read(infile);
    ...

Clearly the `sam_hdr_read` is not within the if-clause of
`hts_set_opt` returning < 0, but it's how it reads because "..."
hid too much.  (In the above case just indenting the ... and
un-indenting the hdr read call reverses the mental image.)

My approach here is to replace all `...` error handlers with `... // error`
and to indent them accordingly.

Also where we only have a one line thing, removing open curly braces
if we've cut the close curly brace gives a more sensible view.

None of this is really changing the code we're showing, but presenting
it in a more obvious manner to those that don't understand what is and
isn't an error case.