ERR: improve error message for invalid indexer
Convert the long repr of the Index into just the name. So something like

```
cannot do slice indexing on <class 'pandas.core.indexes.datetimes.DatetimeIndex'> with these indexers [key] of <class 'float'>
```

becomes

```
cannot do slice indexing on DatetimeIndex with these indexers [key] of type float"
```