Improve <Plug> handling for disabled plugins

[chylex]

Hi, it'd be nice if IdeaVIM showed an error message when trying to use a <Plug> mapping with a plugin that isn't enabled, instead of trying to execute whatever's inside the parentheses. Just spent half an hour trying to figure out why

map <Space><Space> <Plug>(easymotion-prefix)

was literally executing easymotion-prefix as if I typed that instead, and went debugging IdeaVIM until I found the plugin wasn't getting loaded because I forgot set easymotion in .ideavimrc...