Add more context to the non-unique array item in error

[yuri1969]

Resolving validation errors caused by array items defined as unique requires post-processing the result to find the dups.

To illustrate, a validation of an array with 100s of items results in vague error of:

<array> has non-unique elements

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Enriching ValidationError with additional exact context as an index/duplicated item would help a lot.

[Julian]

This isn't likely to happen unless #517 happens, as right now we don't calculate this information (and doing so isn't strictly necessary to know the list contains duplicates). But given that #517 may indeed happen some day I'm happy to leave this open to consider at that point.