Add error messaging to pivot DuplicateValueError failures

[jmuharsky]

This resolves an issue where certain pivot errors were returning an empty string.

[jmuharsky]

Demo is at https://27-1-dot-perfkit-explorer.googleplex.com

[ivansmf]

I suggested a more actionable error message, could you please review?

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[ivansmf]

Dear Googlebot, you're drunk, go home. https://g3doc.corp.google.com/company/teams/opensource/cla/index.md?cl=head#googlers

On Mon, Mar 28, 2016 at 5:59 PM, googlebot notifications@github.com wrote:

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[jmuharsky]

Good otherwise? I updated the error message.