Improve "already exists" error message

[rly]

Motivation

The previous message was not very informative about why adding the container was not allowed.

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.88%. Comparing base (875712b) to head (23668f8).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1165   +/-   ##
=======================================
  Coverage   88.88%   88.88%           
=======================================
  Files          45       45           
  Lines        9835     9835           
  Branches     2795     2795           
=======================================
  Hits         8742     8742           
  Misses        776      776           
  Partials      317      317           

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