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> Is it a performance worry?
It really depends on what people are doing with their down. If indexing potentially missing values is common, there will be a performance impact. 
Also, the cost of a __repr__ varies wildly depending on the data (i.e. a collections.Counter instance has an expensive __repr__, decimal objects have computations to runs as well).
From a user point of view, a repr might be helpful or it might be disasterously lengthy (a webpage, a giant bytearray, a long list of tuples).
Personally, I would rather not do this PR which seems to have the view that the exception is an error condition as opposed to being a normal way to terminate a series of index() calls.