pgvector: Include the actual SQL exception when reporting the error.

[superkelvint]

The pgvector document store does not currently report SQL exceptions, which makes debugging difficult.

Using hayhooks, for example, with this patch, the error goes from this:

│ Server error: Pipeline execution failed: The following component failed to run: │
│ Component name: 'indexing:writer'                                               │
│ Component type: 'DocumentWriter'                                                │
│ Error: Could not write documents to PgvectorDocumentStore.                      │
│ You can find the SQL query and the parameters in the debug logs.                │

to this:

│ Server error: Pipeline execution failed: The following component failed to run:
│ Component name: 'indexing:writer'
│ Component type: 'DocumentWriter'
│ Error: Could not write documents to PgvectorDocumentStore:         │
│ expected 768 dimensions, not 384 
│You can find the SQL query and the parameters in the debug logs."}                   │

Proposed Changes:

Include the SQL exception in the reporting message.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.