message: add Errorf and %w support to formatter

[SamWhited]

This patch adds support for the %w verb and Errorf method similar to the fmt package.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/620515.
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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 2:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/620515.
After addressing review feedback, remember to publish your drafts!