Improve diagnostics when invalid --subtitle specified

[blabzilla]

Hi

Thanks so much for this program. It has made my slog through transferring a DVD set i acquired to my media box a breeze.

I ran into an issue though, when i specified --subtitle=eng, as suggested (probably not literally, though i had no clue) in the --help message. Turns out, my DVD only features (inter alia) "English" (unabbreviated), but autohb only told me:

autohb.rb:462:in `get_subtitle_track': no implicit conversion of String into Integer (TypeError)
        from autohb.rb:654:in `main'
        from autohb.rb:933:in `<top (required)>'

So i modified get_subtitle_track with some code i copy-pasted from get_subtitles to result in the branch i'm requesting being pulled into yours, which, to my illegal --subtitle specification, ponies up

autohb.rb:473:in `get_subtitle_track': no subtitle language "eng" found (for track 1). (RuntimeError)
valid --subtitle= options (for that track, anyway):
["English", "Francais", "español", "Portugues", "ไทย", "English,"]

(I have no idea what that last one, "English," with the comma, is. I'm itchin' to try it, though)

Of course, that's at the top of the call stacktrace, so it may not be all that helpful, and my ruby has never been for public consumption, so it may not be exactly a paragon of style, so you may just wanna take the gist of it and do something more suitable for your purposes, if you see any merit at all in this.

Anyways, again, thanks for this very time-saving program.

And, for the record, i was running the version at commit 890781e of Fri Dec 2 22:05:18 2022 +0000 on ubuntu 22.04 (jammy), which (for anybody else with a box running that distro who hasn't done so yet) i got to build courtesy of an apt-get install ruby-bundler, an apt-get install ruby-dev, and an apt-get install g++ (didn't even have a C++ compiler installed on that box) along with replacing the line in the Gemfile that says

gem 'taglib-ruby'

with

gem 'taglib-ruby', '~> 1.1.3'

which seems to be the version available just after you made that last commit, because the older GNU C++ versions available with "jammy" don't support some of the newer C++ syntax in the latest taglib-ruby version 2.0.0. (I further followed the suggestion in the comment by Iuri Guilherme at Jun 16, 2022 at 15:25 on this stackoverflow answer to bundle install the gems locally in my AutoHandbrake directory and then ran it a la bundle exec autohb.rb)