trapme: No error message when "empty" `UPSTREAM_JOB_NUMBER` specified

Spotted on https://ci.adoptium.net/job/Test_openjdk21_hs_extended.openjdk_riscv64_linux/32/console

That job seems to have failed because it specified an `UPSTREAM_JOB_NUMBER` which no longer had any associated artifacts, but that is not obvious in the log:
```
12:44:17 Timeout set to expire in 1 hr 0 min
[Pipeline] {
[Pipeline] dir
12:44:17 Running in /home/jenkins/workspace/Test_openjdk21_hs_extended.openjdk_riscv64_linux/jdkbinary
[Pipeline] {
[Pipeline] copyArtifacts
[Pipeline] }
[Pipeline] // dir
[Pipeline] }
[Pipeline] // timeout
[Pipeline] }
[Pipeline] // stage
[Pipeline] echo
12:44:17 PROCESSCATCH: Terminating any hung/left over test processes:
[Pipeline] sh
12:44:18 + aqa-tests/terminateTestProcesses.sh jenkins
12:44:18 Unix type machine..
12:44:18 Running on a Linux host
12:44:18 Woohoo - no rogue processes detected!
[Pipeline] cleanWs
12:44:18 [WS-CLEANUP] Deleting project workspace...
```

We should spot this situation and give a suitable error message to make this scenario easier to identify and debug so as not to spend too much time trying to diagnose it.

Start by tracing the pipeline's copyArtifacts step and how it handles UPSTREAM_JOB_NUMBER when no artifacts exist. Reproduce the missing-artifact case from the linked console output and confirm that the completed work reports a clear error identifying the unavailable upstream job.

Written by the indexing model from the issue text.