chore(stresstest): Add better error output

[jan-auer]

Print errors with their cause so we see why requests are failing with more
information when running stresstests.

Example output:

ERROR: writing object
  caused by: error writing payload
  caused by: error sending request for url (http://localhost:18888/v1/objects/slow/org=0;project=0/)
  caused by: operation timed out