[gdb/python] Handle failure to initialize without exiting

[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>