[Ag] Print error message when ag is not found on $PATH

Close #440

autoload/fzf/vim.vim

@@ -649,6 +649,9 @@ endfunction
 
 " ag command suffix, [options]
 function! fzf#vim#ag_raw(command_suffix, ...)
+  if !executable('ag')
+    return s:warn('ag is not found')
+  endif
   return call('fzf#vim#grep', extend(['ag --nogroup --column --color '.a:command_suffix, 1], a:000))
 endfunction