rpc: add user name to signUser error message
rpc: add user name to signUser error message

Change-Id: I00d3f0f995115cd208e6efce95d15cd157e82e47
Reviewed-on: https://upspin-review.googlesource.com/14180
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin