TOOLS/zsh.pl: die if we can't parse main options

This will catch cases where mpv runs without error, but the
--list-options output isn't what we expect. Otherwise, we'll make a
broken completion file that will result in cryptic errors when pressing
tab, like:

_mpv:18: command not found: *:files:->mfiles

That's been the case for most of the zsh completion issues we've had
reported, that I can remember.

Also make uninitialized variable access fatal so that failures to parse
other options will also make the script die eventually, albeit with a
less nice message.

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