Include positions filename in the error when YAML unmarshal fails.

[pstibrany]

When reading of positions file failed previously, error message looked like this:

level=error ts=2019-11-25T07:53:35.626673Z caller=main.go:61 msg="error creating promtail" error="yaml: line 3: found unexpected end of stream"

Without further context, it's not clear that the problem is in positions file, and one may incorrectly assume that it's the config file that is wrong. This PR changes error message to include the filename:

level=error ts=2019-11-25T08:30:07.944456Z caller=main.go:61 msg="error creating promtail" error="/tmp/1187-positions.yml: yaml: line 3: found unexpected end of stream"

This PR also changes error message when loading of config file fails from...

Unable to parse config: sourcing: yaml: line 54: found unexpected end of stream

to

Unable to parse config: 1187-promtail.yaml: yaml: line 54: found unexpected end of stream

[sh0rez]

LGTM

[cyriltovena]

LGTM