serverutil/upspinserver: fix error handling for config homedir
serverutil/upspinserver: fix error handling for config homedir

It looks to me like the error logic was reversed.

Also, since mistakes by either us or server admins may be mysterious,
slightly improve the message in case the config file is not found.

Change-Id: Ib797e73651720f1e07312a42142bfc4a2f99b352
Reviewed-on: https://upspin-review.googlesource.com/11580
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

upspin/upspin,upspin/upspin