Clean-up error after trying to draw overlay on unmodifiable (terminal) buffer

[simlei]

Hi,
the terminal that was introduced in vim 8 apparently gives Plugins like yours a headache when they try to draw on the buffer. In the case of vim-choosewin, the error is handled kind of gracefully and switching still works, just on the terminal buffer, no overlay is drawn, and an error appears below in a non-intrusive mesage. Cheers -- that is handled better by far than the popular "easymotion" plugin.

Still, the status bars of the windows retain the "A"to"Z" markers after this error occured. some error handling or buffer type checking is needed. I am sorry to say that I don't know the first thing about it as I just started with vim.

Thanks again for the great plugin.

[t9md]

I'm now not active vim user(I use Atom), so I can't fix it by myself.
But I can check and merge PR for fixing this issue.

Thanks.