[gdb/python] Use gdbpy_handle_gdb_exception in valpy_assign_core

In valpy_assign_core we have:
...
  catch (const gdb_exception &except)
    {
      gdbpy_convert_exception (except);
      return false;
     }
...

Use instead:
...
  catch (const gdb_exception &except)
    {
      return gdbpy_handle_gdb_exception (false, except);
    }
...

No functional changes.

Tested on x86_64-linux.

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