cmd/cacheserver: change the returned error chan to uni-direction
cmd/cacheserver: change the returned error chan to uni-direction

This is a very tiny little change for me to get familiar with upspin
contribution workflow.

This commit makes it clear that the caller should only receive error
item from the returned chan.

Change-Id: I2d152a1ed17e93926372115ae8aa3aec244c7717
Reviewed-on: https://upspin-review.googlesource.com/8545
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

upspin/upspin,upspin/upspin