[gdb/python] Make gdb.UnwindInfo.add_saved_register more robust (fixup)

In commit 2236c5e384d ("[gdb/python] Make gdb.UnwindInfo.add_saved_register
more robust") I added this code in unwind_infopy_add_saved_register:
...
  if (value->optimized_out () || !value->entirely_available ())
...
which may throw c++ exceptions.

This needs to be caught and transformed into a python exception.

Fix this by using GDB_PY_HANDLE_EXCEPTION.

Tested on x86_64-linux.

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

Fixes: 2236c5e384d ("[gdb/python] Make gdb.UnwindInfo.add_saved_register more robust")