Fix custom rule error handling

[Andyz26]

Context

• Remove not used runtime exception.
• UT to validate error behavior: run failure from custom trigger should NOT trigger rule actor restart to avoid short-circuit.

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

646 tests  +1   636 ✅ +1   8m 41s ⏱️ +6s
149 suites ±0    10 💤 ±0 
149 files   ±0     0 ❌ ±0 

Results for commit 63670fc. ± Comparison against base commit 8489569.

This pull request removes 1 and adds 2 tests. Note that renamed tests count towards both.

io.mantisrx.server.worker.jobmaster.rules.CustomRuleActorTest ‑ testBasic

io.mantisrx.server.worker.jobmaster.rules.CustomRuleActorTest ‑ testHappyRun
io.mantisrx.server.worker.jobmaster.rules.CustomRuleActorTest ‑ testRunError