misc/dist: check return value from filepath.Walk
misc/dist: check return value from filepath.Walk

Doesn't fix any known issue. This bit me in some unrelated
code and I thought of this tool.

R=golang-dev, krautz, mikkel
CC=golang-dev
http://codereview.appspot.com/5976067

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