go.tools/cover: validate flags with appropriate error messages
go.tools/cover: validate flags with appropriate error messages

Fixes issue 6444.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13956043

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

lattera/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,lattera/go.tools,karmakaze/go.tools,chenjianlong/go.tools,lattera/go.tools,chenjianlong/go.tools,karmakaze/go.tools