Better error message for non-increasing index.

[IvoDD]

Attempting to write a non-increasing index version is most likely due to parallel writes to the same symbol. We now indicate that in the error message.

Reference Issues/PRs

What does this implement or fix?

Any other comments?

Checklist

Checklist for code changes...

• [ ] Have you updated the relevant docstrings, documentation and copyright notice?
• [ ] Is this contribution tested against all ArcticDB's features?
• [ ] Do all exceptions introduced raise appropriate error messages?
• [ ] Are API changes highlighted in the PR description?
• [ ] Is the PR labelled as enhancement or bug so it appears in autogenerated release notes?