cacheserver: print less ambiguous message in cacheserver.Start
cacheserver: print less ambiguous message in cacheserver.Start

Certain processes (upspin, upspinfs) start cacheserver as needed.
If cacheserver does not start or terminates, an error
message is printed. The old message implied that we were
trying to restart it, which wasn't true.

Change-Id: I28b5c9743008c933d6dfe1a156016783f23d14ac
Reviewed-on: https://upspin-review.googlesource.com/18500
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin