Defer warning about incompatible Python.

This uses the GundoDidNotLoad approach from 8de0b4ce47 also
for the "Gundo requires that Vim be compiled with Python 2.4+" error.

Also retab the original GundoDidNotLoad code block and fix the English.

plugin/gundo.vim

@@ -33,15 +33,15 @@ import vim
 if sys.version_info[:2] < (2, 4):
     vim.command('let s:has_supported_python = 0')
 ENDPYTHON
-
-    " Python version is too old
-    if !s:has_supported_python
-        echo  "Gundo requires that Vim be compiled with Python 2.4+"
-        finish
-    endif
 else
-    " no Python support
+    let s:has_supported_python = 0
-    echo  "Gundo requires that Vim be compiled with Python 2.4+"
+endif
+
+if !s:has_supported_python
+    function! s:GundoDidNotLoad()
+        echohl WarningMsg|echomsg "Gundo requires Vim to be compiled with Python 2.4+"|echohl None
+    endfunction
+    command! -nargs=0 GundoToggle call s:GundoDidNotLoad()
     finish
 endif"}}}