test: make load test err msg explicit

[btlghrants]

Description

Overnight load test failed just as it was beginning -- 2.5 min into a 3 hr expected runtime. Diagnosing failure let to discovery of an unhelpful message: The promise rejected with the reason "#<Object>".

This PR adds the catch block necessary to see what actually went wrong.

Load test start: 2024-11-22T05:09:47.892Z
{
  "module": "/home/runner/work/pepr/pepr/pepr-excellent-examples/hello-pepr-load",
  "manifest": "/home/runner/work/pepr/pepr/pepr-excellent-examples/hello-pepr-load/capabilities/configmap.yaml"
}
{
  "actInterval": "1m",
  "actIntensity": 4000,
  "audInterval": "1s",
  "clusterName": "pepr-load-cluster",
  "duration": "1h",
  "outputDir": "/home/runner/work/pepr/pepr/pepr/load",
  "settle": "2h",
  "stagger": "5s"
}

UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "#<Object>".
    at throwUnhandledRejectionsMode (node:internal/process/promises:389:7)
    at processPromiseRejections (node:internal/process/promises:470:17)
    at processTicksAndRejections (node:internal/process/task_queues:96:[32](https://github.com/defenseunicorns/pepr/actions/runs/11966824488/job/33363083364#step:16:33)) {
  code: 'ERR_UNHANDLED_REJECTION'
}
↓
Error: Process completed with exit code 1.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Checklist before merging

• Unit, Journey, E2E Tests, docs, adr added or updated as needed
• Contributor Guide Steps followed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.56%. Comparing base (d2dec12) to head (2b0f82d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1478   +/-   ##
=======================================
  Coverage   78.56%   78.56%           
=======================================
  Files          37       37           
  Lines        1717     1717           
  Branches      410      402    -8     
=======================================
  Hits         1349     1349           
+ Misses        366      339   -27     
- Partials        2       29   +27     

see 7 files with indirect coverage changes

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