promu exits with 0 for bad config

[gouthamve]

See: https://circleci.com/gh/prometheus/prometheus/8181

Using config file: /home/circleci/.go_workspace/src/github.com/prometheus/prometheus/.promu.yml
/!\ unknown/unhandled platforms: [linux/s390x]
github.com/prometheus/promu/cmd.runCrossbuild
	/home/circleci/.go_workspace/src/github.com/prometheus/promu/cmd/crossbuild.go:128
github.com/prometheus/promu/cmd.glob..func5
	/home/circleci/.go_workspace/src/github.com/prometheus/promu/cmd/crossbuild.go:62
github.com/prometheus/promu/vendor/github.com/spf13/cobra.(*Command).execute
	/home/circleci/.go_workspace/src/github.com/prometheus/promu/vendor/github.com/spf13/cobra/command.go:653
github.com/prometheus/promu/vendor/github.com/spf13/cobra.(*Command).ExecuteC
	/home/circleci/.go_workspace/src/github.com/prometheus/promu/vendor/github.com/spf13/cobra/command.go:728
github.com/prometheus/promu/vendor/github.com/spf13/cobra.(*Command).Execute
	/home/circleci/.go_workspace/src/github.com/prometheus/promu/vendor/github.com/spf13/cobra/command.go:687
github.com/prometheus/promu/cmd.Execute
	/home/circleci/.go_workspace/src/github.com/prometheus/promu/cmd/promu.go:64
main.main
	/home/circleci/.go_workspace/src/github.com/prometheus/promu/main.go:20
runtime.main
	/usr/local/go/src/runtime/proc.go:185
runtime.goexit
	/usr/local/go/src/runtime/asm_amd64.s:2337\n> running the base builder docker image
 +  docker run --rm -t -v /home/circleci/.go_workspace/src/github.com/prometheus/prometheus:/app quay.io/prometheus/golang-builder:1.10-base -i github.com/prometheus/prometheus -p linux/amd64 linux/386 darwin/amd64 darwin/386 windows/amd64 windows/386 freebsd/amd64 freebsd/386 openbsd/amd64 openbsd/386 netbsd/amd64 netbsd/386 dragonfly/amd64 linux/armv5 linux/armv6 linux/armv7 linux/arm64 freebsd/armv6 freebsd/armv7 netbsd/armv6 netbsd/armv7 linux/ppc64 linux/ppc64le

This refers to this PR: prometheus/prometheus#4605 where build succeeded even with bad config.

[sdurrheimer]

It exits with a code 0 because it's a warning, not a error.

[SuperQ]

@gouthamve Is this still a problem?