Handle error on nvim_buf_get_extmarks

[lucasdf]

I am getting this error when closing the autocomplete buffer/popup:

Error executing vim.schedule lua callback: ...te/pack/packer/start/hologram.nvim/lua/hologram/init.lua:47: Invalid buffer id: 7                                                                                                                                                                                        
stack traceback:
        [C]: in function 'nvim_buf_get_extmarks'
        ...te/pack/packer/start/hologram.nvim/lua/hologram/init.lua:47: in function 'buf_render_images'
        ...te/pack/packer/start/hologram.nvim/lua/hologram/init.lua:17: in function <...te/pack/packer/start/hologram.nvim/lua/hologram/init.lua:17>

It seems nvim_buf_get_extmarks can throw an error in case the buffer doesn't exist anymore, so it might be a good idea to skip it in case it throws.