fix: improve missing rplugin error handling; less agressive tracebacks

Avoid exceptions inside the FileType python autocmd, because the
stacktrace is ugly and often difficult to read.

Instead, show an asynchronous notification that something is broken
and users should run `:UpdateRemotePlugins`.

plugin/semshi.vim

@@ -43,11 +43,27 @@ function! s:remove_builtin_extra()
     hi link pythonKeyword pythonNumber
 endfunction
 
+" Ensure the rplugin manifest
 if exists(':Semshi') == 0
   command! -nargs=* Semshi call nvim_err_writeln(":Semshi not found. Run :UpdateRemotePlugins.")
+
+  let s:semshi_rplugin_error = 1
+  if exists(':lua') && has('nvim-0.5.0') > 0
+lua << EOF
+    vim.schedule(function()
+      vim.notify(":Semshi not found. Run :UpdateRemotePlugins.", 'ERROR', { title = "semshi" })
+    end)
+EOF
+  endif
 endif
 
 function! s:filetype_changed()
+    if exists('s:semshi_rplugin_error')
+      " Avoid exceptions inside FileType autocmd, because the stacktrace is ugly.
+      " Instead, an asynchronous notification that something is broken will be made.
+      return
+    end
+
     let l:ft = expand('<amatch>')
     if index(g:semshi#filetypes, l:ft) != -1
         if !get(b:, 'semshi_attached', v:false)