key/server: fix cache invalidation
key/server: fix cache invalidation

We should invlidate positive and negtive caches on error.
Returning an error does not necessarily mean that the Put
fails on the server side. The connection might fail during
the client receiving the response from the server.

Change-Id: I97704c4020271c57a2c6c44d6ba4d28b626a873f
Reviewed-on: https://upspin-review.googlesource.com/8559
Reviewed-by: Eduardo Pinheiro <c9d6fd76b920b269b838bbb70f683c6db503d8d1@gmail.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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