Add hclose_abruptly() for stack unwinding after errors

This variant of hclose() is for use when an (e.g. I/O) error has already
occurred and you just need to close and deallocate the stream without
caring about any further errors.
Change hts_close() to return int (error indication)

Also add documentation to hts_close() and reformat hts_open() docs and
note format autodetection when reading.

Fixes #29.  Further changes are likely in the places where hts_close()
is called, to report errors.