Throw exception when doing e.g. Lookup.Smaller on unsorted frame

[tpetricek]

If I have an unordered series and I write:

frame.Rows |> Series.tryLookup key Lookup.Smaller

... then this always returns None. I think there are two reasonable things to do:

• Throw an exception, because inexact lookup is only supported on ordered series
• In principle, this could just return the value at offset - 1 where offset is the offset of key, but this would only work when the key actually exists in the frame, so I think the first option makes more sense.

[hmansell]

I agree.