ssh: improve error message for KeyboardInteractiveChallenge

[packrat386]

Fixes golang/go#41823

[packrat386]

I've not contributed to this or other go repos before. Did my best to follow the guidelines for new contributors, but let me know if there's anything that I need to add or change.

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This PR (HEAD: 179b3ee) has been imported to Gerrit for code review.

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Patch Set 1:

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Message from Filippo Valsorda:

Patch Set 1: Run-TryBot+1 Code-Review+2

(3 comments)

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Message from Gobot Gobot:

Patch Set 1:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=10ec7661

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Patch Set 1: TryBot-Result+1

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Message from Aidan Coyle:

Patch Set 2:

(3 comments)

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Message from Roland Shoemaker:

Patch Set 2: Trust+1

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Message from Go Bot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://golang.org/doc/contribute.html#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11 or adds a tag like "wait-release", it means that this CL will be
reviewed as part of the next development cycle. See https://golang.org/s/release
for more details.

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Patch Set 1:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=10ec7661

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Patch Set 1: TryBot-Result+1

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Message from Filippo Valsorda:

Patch Set 2: Run-TryBot+1

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Message from Go Bot:

Patch Set 2:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=e0b50283

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Message from Go Bot:

Patch Set 2: TryBot-Result-1

1 of 10 TryBots failed:
Failed on android-amd64-emu: https://storage.googleapis.com/go-build-log/af874805/android-amd64-emu_45c9e72d.log

Consult https://build.golang.org/ to see whether they are new failures. Keep in mind that TryBots currently test exactly your git commit, without rebasing. If your commit's git parent is old, the failure might've already been fixed.

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Patch Set 2: TryBot-Result+1

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This PR is being closed because golang.org/cl/259902 has been merged.