net/http: improve TestServerExpect
net/http: improve TestServerExpect

Fail more usefully, and Logf in one place instead of Errorf where
an error is acceptable.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6221059

tv42/old-go,apparentlymart/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,gnanderson/go,dsriram/golang,gnanderson/go,dsriram/golang,gnanderson/go,apparentlymart/golang,dsriram/golang