http2: improve error when server sends HTTP/1

[colega]

If for some reason the server sends an HTTP/1.1 response
starting with "HTTP/1.1 " (9 bytes), http2 transport interprets that as
a valid frame header for an expected payload of ~4MB, and fails with
non-descriptive messages like "unexpected EOF".

This could happen, for example, if ALPN is misconfigured on the server's
load balancer.

This change attempts to improve that feedback by noting in the error
messages whenever the frame header was exactly the "HTTP/1.1 " bytes.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Oleg Zaytsev:

Patch Set 1:

(2 comments)

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[gopherbot]

Message from Damien Neil:

Patch Set 3:

(1 comment)

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Message from Damien Neil:

Patch Set 3:

(1 comment)

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[gopherbot]

Message from Oleg Zaytsev:

Patch Set 3:

(2 comments)

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[gopherbot]

This PR (HEAD: 4abf41d) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/net/+/623155.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
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• To change your code in response to feedback:
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[gopherbot]

Message from Oleg Zaytsev:

Patch Set 3:

(2 comments)

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[gopherbot]

Message from Oleg Zaytsev:

Patch Set 3:

(1 comment)

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Message from Damien Neil:

Patch Set 4: Code-Review+2

(1 comment)

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