[gdb/python] Handle !Py_IsInitialized () in gdbpy_initialize

I tried out making python initialization fail by passing an incorrect
PYTHONHOME, and got:
...
$ PYTHONHOME=foo gdb -q
Python path configuration:
  PYTHONHOME = 'foo'
  ...
Python Exception <class 'ModuleNotFoundError'>: No module named 'encodings'
Python not initialized
$
...

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

   gdbpy_enter enter_py;
...

What happens is that:
- do_start_initialization returns false because Py_InitializeFromConfig fails,
  leaving us in the !Py_IsInitialized () state
- PyErr_Occurred () returns true
- gdbpy_print_stack is called, which prints
  "Python Exception <class 'ModuleNotFoundError'>: No module named 'encodings"

The problem is that in the Py_IsInitialized () == false state, very few
functions can be called, and PyErr_Occurred is not one of them [1], and
likewise functions in gdbpy_print_stack.

Fix this by:
- guarding the PyErr_Occurred / gdbpy_print_stack part with Py_IsInitialized ().
- handling the !Py_IsInitialized () case by printing the failure PyStatus
  in do_start_initialization

This gets us instead:
...
$ 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
$
...

Tested on aarch64-linux.

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

[1] https://docs.python.org/3/c-api/init.html#before-python-initialization

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