DOC: limit error traceback to one line for expected exceptions (GH10715)
Closes #10715

This gives you an output like

```
In [4]: s = pd.Series([1,3,5,np.nan,6,8])

In [5]: s.iloc[11]
---------------------------------------------------------------------------
IndexError: single positional indexer is out-of-bounds
```

for input

```
.. ipython:: python
   :okexcept:

   s = pd.Series([1,3,5,np.nan,6,8])
   s.iloc[10]
```

I think we would always want such a truncated traceback in the docs in cases that exceptions are expected, so I don't think it is needed to make it an extra new option. 
I only should check for cases where the actual message spans multiple lines (now it is hardcoded to give me the last line of the error traceback)

@jreback with this we don't have to use the `code-block` s in eg whatsnew for the expect behaviour of `to_datetime`
