Transforming a frame to itself should raise an exception if an explicit self-transform isn't defined

i.e:
```python
In [2]: HeliographicStonyhurst(10*u.deg, 10*u.deg, obstime="2011-02-01").transform_to(HeliographicStonyhurst(obstime="2011-01-01"))
Out[2]:

```
Should error as there is no transform in the graph for `HeliographicStonyhurst` > `HeliographicStonyhurst`

Start by reproducing the HeliographicStonyhurst example and tracing the transform_to entry point and transformation-graph lookup it exercises. Done means an implicit self-transform raises an exception when no explicit self-transform is defined, while the issue's example no longer returns a coordinate silently.

Written by the indexing model from the issue text.