I don't think it's necessary to `command go`. If Go is installed, `go version` will show its version. If it's not installed, bash will output "go: command not found" followed by our message that you need to install Go 1.2, which is enough for you to know what to do next.

I also don't think we should be checking `$GOPATH`. I wrote `script/build` to work even when there is no GOPATH set at all.

Yep, this PR is all about reducing the painful experience. If a new user gets an error message they don't know what to do about, we have failed.