proper support for kohlchan.net

[SoerenLauert]

CTW 1.17.1

in general the tool works with kohlchan.net but the site uses double file names. the original one from the uploaded file and the chan sequence number. in consequence CTW downloads every file twice. a fix would be appreciated.

there is also the occasional case where a thread download stops with an unknown error. while all the files seem to be there in the download folder, the html file is messed up with missing images and wrong formatting. and the tool stops updating the thread of course if this error occurs.