Clarify error message when no valid target scrape config is defined for promtail job

[dannykopping]

What this PR does / why we need it:
I was recently trying to understand why I was getting this error message:

unable to create logs instance: unknown scrape config when using the Grafana Agent with this config:

...loki:
  configs:
      ...
      name: default
      scrape_configs:
        - job_name: my-job-name
          pipeline_stages:
            - multiline:
                firstline: '^\d{4}-\d{2}-\d{2}\s\d{1,2}\:\d{2}\:\d{2}\.\d{3}'
...

The problem here is not that the scrape config is unknown, but rather that there is no target scrape config defined for this particular job.

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:
Slack thread with one of our Solutions Engineers

Checklist

• Documentation added
• Tests updated