rpc: clarify error when another dune instance is running

[meet0806]

Summary

• improve forwarded-command error when another dune instance holds the global lock but RPC endpoint is unavailable
• emit a clearer user-facing message: "Another Dune instance is currently running. Aborting..."
• keep behavior deterministic (no pid in message) for stable cram expectations
• update cram test expectation in custom-build-dir case

Validation

• opam exec -- ./dune.exe runtest test/blackbox-tests/test-cases/cram/custom-build-dir.t

Fixes #13902