Add log to indicate the source job cluster is absent

[hellolittlej]

Context

• when source job cluster is absent, we simply returns a dummy SSE event without logging
• the downstream job would restart once dummy source job observable completes
• the job get stuck at restart loop if a stale source exists and job cluster is removed

Checklist

• ./gradlew build compiles code correctly
• Added new tests where applicable
• ./gradlew test passes all tests
• Extended README or added javadocs where applicable

[github-actions]

Test Results

164 files  ±0  164 suites  ±0   11m 19s ⏱️ +29s
795 tests ±0  783 ✅ ±0  11 💤 ±0  1 ❌ ±0 
796 runs  ±0  784 ✅ ±0  11 💤 ±0  1 ❌ ±0 

For more details on these failures, see this check.

Results for commit f7f1051. ± Comparison against base commit ee6af0c.

♻️ This comment has been updated with latest results.