If this slice ends up empty after parsing the HTTP response body, it
means the body is not well-formed. We've probably encountered an error
message produced by something that uses a different JSON schema, or
an error that just happens to validate as JSON.
An empty errcode.Errors slice is not a very useful thing to return,
since its Error() output is just `<nil>`. Detect this case, and instend
return an UnexpectedHTTPResponseError.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
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