rpc: remove invalid argument to errors.E
rpc: remove invalid argument to errors.E

upspin.NetAddr is not a type errors.E understands.

Change-Id: I65d51c227d3e85f88c8c89c95faf9f488b6b8577
Reviewed-on: https://upspin-review.googlesource.com/7645
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>

upspin/upspin,upspin/upspin