ERR: improve error message for invalid indexer

[jorisvandenbossche]

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"

[jorisvandenbossche]

Since this is for iloc, changing to positional (otherwise you can get confusing error message as "cannot do slice indexing on FloatIndex with indexer of type float" (which for label-based slice indexing would of course be totally fine)

[WillAyd]

lgtm