fix: handle INIT from frontend in running scheduler which is not in shouldRun state

[mxab]

What this PR does / why we need it:

We expirience the situation where the scheduler is not yet in a "shouldRun" state but receives an init message from the frontend.
In this case the scheduler will not reply with an OK but rather will just wait for the next message

This means the frontend worker is then blocked as it waits for any kind of reply from the scheduler.
Therefore the frontend worker connects to the scheduler but does not forward incoming queries

Which issue(s) this PR fixes:

Fixes #19528

This PR mitigates this issue by sending as the scheduler now sends an ERROR in case it's RUNNING but shouldRun is still false

This PR is the "smaller" version of this PR as I hope its easier to review while the other one also add an additional timeout on the frontend side

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR