[gdb/python] Handle failure to initialize without exiting

I tried out making python initialization fail by passing an incorrect
PYTHONHOME, and got:
...
$ PYTHONHOME=foo ./gdb.sh -q
Python path configuration:
  PYTHONHOME = 'foo'
  ...
Python initialization failed: \
  failed to get the Python codec of the filesystem encoding
Python not initialized
$
...

The relevant part of the code is:
...
static void
gdbpy_initialize (const struct extension_language_defn *extlang)
{
  if (!do_start_initialization () && py_isinitialized && PyErr_Occurred ())
    gdbpy_print_stack ();

   gdbpy_enter enter_py;
...

What happens is:
- gdbpy_enter::gdbpy_enter () is called, where we run into:
  'if (!gdb_python_initialized) error (_("Python not initialized"));'
- the error propagates to gdb's toplevel
- gdb print the error and exits.

It seems unnecesssary that we exit gdb.  We could continue the
session without python support.

Fix this by:
- bailing out of gdbpy_initialize if !do_start_initialization
- bailing out of finalize_python if !gdb_python_initialized

This gets us instead:
...
$ PYTHONHOME=foo gdb -q
Python path configuration:
  PYTHONHOME = 'foo'
  ...
Python initialization failed: \
  failed to get the Python codec of the filesystem encoding
(gdb) python print (1)
Python not initialized
(gdb)
...

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/python/python.c

@@ -2215,6 +2215,9 @@ static struct cmd_list_element *user_show_python_list;
 static void
 finalize_python (const struct extension_language_defn *ignore)
 {
+  if (!gdb_python_initialized)
+    return;
+
   struct active_ext_lang_state *previous_active;
 
   /* We don't use ensure_python_env here because if we ever ran the
@@ -2766,8 +2769,21 @@ do_initialize (const struct extension_language_defn *extlang)
 static void
 gdbpy_initialize (const struct extension_language_defn *extlang)
 {
-  if (!do_start_initialization () && py_isinitialized && PyErr_Occurred ())
-    gdbpy_print_stack ();
+  if (!do_start_initialization ())
+    {
+      if (py_isinitialized)
+	{
+	  if (PyErr_Occurred ())
+	    gdbpy_print_stack ();
+
+	  /* We got no use for the Python interpreter anymore.  Finalize it
+	     ASAP.  */
+	  Py_Finalize ();
+	}
+
+      /* Continue with python disabled.  */
+      return;
+    }
 
   gdbpy_enter enter_py;