In order to be more consistent with different subtitle formats.
From:
* --write-srt to --write-sub
* --only-srt to --only-sub
* --all-srt to --all-subs
* --srt-lang to --sub-lang'
Refactored also all the mentions of srt for sub in all the source code.
When a query results of 0 items, the key items is not present in the
api_response dictionary, raising a KeyError.
Intead, look for the key and call trouble if it's not present.