Simplify conversion where existing Python object argument expected (fixes issue #67) * __py_struct_from_dict__.cc (F__py_isinstance__, F__py_struct_from_dict__): Use pytave::pyobject_unwrap_object and simplify surrounding code. * oct-py-types.cc (pytave::extract_py_scalar_map): Raise Octave errors instead of throwing exceptions, reword error messages. * @pyobject/pyobject.m: Add %!error tests for struct conversion failure.