[dev.seq] dir/server/tree: fix a couple of errors.E calls
[dev.seq] dir/server/tree: fix a couple of errors.E calls

Now that Tree.user is a type (serverlog.User) rather than an upspin.UserName,
we need to update the calls to errors.E to use user.Name() so the errors are
handled correctly.

Just an oversight from previous changes.

Change-Id: I9e9764fac9367045a35c51cf79a7337a6b1f3702
Reviewed-on: https://upspin-review.googlesource.com/13842
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

upspin/upspin,upspin/upspin