feat: show more informative errors when rplugins were not registered

Many users who are not familiar with how neovim and rplugins works often
get lost on the error 'Semshi: command not found and keeps asking the
same question (e.g., numirias#74), although the easy solution
that one should run `:UpdateRemotePlugins` is already documented.

Nevertheless we can show more informative, fool-proof error messages on
`:Semshi` when remote plugins are not registered or broken somehow.

plugin/semshi.vim

@@ -43,6 +43,10 @@ function! s:remove_builtin_extra()
     hi link pythonKeyword pythonNumber
 endfunction
 
+if exists(':Semshi') == 0
+  command! -nargs=* Semshi call nvim_err_writeln(":Semshi not found. Run :UpdateRemotePlugins.")
+endif
+
 function! s:filetype_changed()
     let l:ft = expand('<amatch>')
     if index(g:semshi#filetypes, l:ft) != -1