rpc: include usernames in proxy authentication error message
rpc: include usernames in proxy authentication error message

Change-Id: I046fbb29d8078e3cb864befe4ae1a1fc4f578004
Reviewed-on: https://upspin-review.googlesource.com/13665
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

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