dir/server/tree: add more error checking
dir/server/tree: add more error checking

Change-Id: I4f35ab8af371f85c9b4ee432f39d5b5fdedf010a
Reviewed-on: https://upspin-review.googlesource.com/11001
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

upspin/upspin,upspin/upspin