client: add the original path name to an error returned by Lookup
client: add the original path name to an error returned by Lookup

One call to errors.E in client.lookup failed to report the original path name.
Provide it so we actually see what the name was that we couldn't find.

Change-Id: If2b6f5265a95b67472f4d901f44a3bb75c830555
Reviewed-on: https://upspin-review.googlesource.com/16960
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

upspin/upspin,upspin/upspin