Add an option to frame_transform_graph.lookup_name to raise error

The ``frame_transform_graph.lookup_name`` function is very helpful, but it would be nice to raise an exception if the frame is not known. At the moment it simply returns ``None``:

```python
In [18]: frame_transform_graph.lookup_name('banana')

In [19]:
```

If it would likely break too many use cases to raise an exception by default, it would be nice to at least add a keyword argument to control this.

cc @cdeil

Start by locating frame_transform_graph.lookup_name and its existing tests, then verify how an unknown frame such as 'banana' is handled. Done means callers can request an exception for unknown frames through an option, with the existing default behavior considered and covered by tests.

Written by the indexing model from the issue text.