populate the location list when :Errors is called

The location list must be set when calling :Errors since our cached
error list is associated with the buffer, but the vim loclist is
associated with the window.

So if the user closes a window, the vim loclist will get destroyed, but
our error list will still exist.

Closes #263

plugin/syntastic.vim

@@ -365,6 +365,7 @@ endfunction
 "display the cached errors for this buf in the location list
 function! s:ShowLocList()
     if !empty(s:LocList())
+        call setloclist(0, s:LocList())
         let num = winnr()
         exec "lopen " . g:syntastic_loc_list_height
         if num != winnr()