promtool: print file/line locations for warnings/errors

[juergenhoetzel]

Checking my config I get warnings, but not the file-name + line-number which cause the warnings/errors:

→ ./promtool  check-config /srv/prometheus/prometheus.yml
Checking /srv/prometheus/prometheus.yml
WARN[0000] The 'target_groups' option for scrape configurations is deprecated, use 'static_configs' instead  source=config.go:468
WARN[0000] The 'target_groups' option for scrape configurations is deprecated, use 'static_configs' instead  source=config.go:468
WARN[0000] The 'target_groups' option for scrape configurations is deprecated, use 'static_configs' instead  source=config.go:468
  SUCCESS: 1 rule files found

These additional info could also improve editor support. I use Emacs and implementing a
Flycheck checker should be easy when this info is provided.

[fabxc]

I agree that it would be handy but with the YAML parsing package we are using this is hardly doable.

[serathius]

I created issue for yaml package go-yaml/yaml#343