mark_errorbar silently fails

```
import pandas as pd
import altair as alt
from altair_transform import transform_chart

df = pd.read_json('https://vega.github.io/vega-datasets/data/iris.json')

error_bars = alt.Chart(df).mark_errorbar(extent='ci').encode(
x=alt.X('petalLength:Q', scale=alt.Scale(zero=False)),
y=alt.Y('species:N')
)

transform_chart(error_bars)
```

The transformation does not take place but `NotImplementedError` is also not raised (which would be expected based on the Limitations section in the README).