I removed the exception handling of the PluginError, because validation error messages are useful now.
Mobile HLS
$ streamlink 'https://mobile.ok.ru/live/1366902775399'
[cli][info] Found matching plugin okru for URL https://mobile.ok.ru/live/1366902775399
Available streams: 240p (worst), 360p, 480p, 720p (best)
Mobile HTTP
$ streamlink 'https://mobile.ok.ru/dk?st.cmd=movieLayer&st.groupId=70000000124591&st.discId=4242734320303&st.retLoc=top&st.rtu=%2Fdk%3Fst.cmd%3DuserMovies%26st.page%3D1%26_prevCmd%3DuserMovies%26tkn%3D8051&st.discType=GROUP_MOVIE&st.mvId=4242734320303&_prevCmd=userMovies&tkn=3276#'
[cli][info] Found matching plugin okru for URL https://mobile.ok.ru/dk?st.cmd=movieLayer&st.groupId=70000000124591&st.discId=4242734320303&st.retLoc=top&st.rtu=%2Fdk%3Fst.cmd%3DuserMovies%26st.page%3D1%26_prevCmd%3DuserMovies%26tkn%3D8051&st.discType=GROUP_MOVIE&st.mvId=4242734320303&_prevCmd=userMovies&tkn=3276#
Available streams: vod (worst, best)
Main site:
$ streamlink 'https://ok.ru/video/266205792931'
[cli][info] Found matching plugin okru for URL https://ok.ru/video/266205792931
Available streams: 144p (worst), 240p, 360p, 480p (best)