update an outdated error message in DDPPlugin

[awaelchli]

What does this PR do?

Fixes #8998

Replaces an outdated error message saying that fit() is not allowed to be called multiple time.
The new error message addresses a assertion error with a friendly message with possible causes of the malfunction.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

I made sure I had fun coding 🙃

[codecov]

Codecov Report

Merging #9005 (b2ef688) into master (e9ce598) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9005    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         175     175            
  Lines       14466   14463     -3     
=======================================
- Hits        13414   12812   -602     
- Misses       1052    1651   +599     

[tchaton]

LGTM !