all: replace calls to errors.As with errors.AsType

[jub0bs]

This change eliminates the remaining calls to errors.As outside the
errors package. CL 708495 purported to eliminate all such calls but
missed a few, and a couple had since popped up.

[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/733400.
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[gopherbot]

Message from jub0bs:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/733400.
After addressing review feedback, remember to publish your drafts!