vstreamer: improve error message when table not found due to keyspace…

[varundeepsaini]

Description

Improves error message when VReplication fails to find a table due to keyspace vindex errors. Previously, if a region_json vindex failed to load its config file (not mounted on vttablet), users saw only "table customer not found". Now the underlying error is included: table customer not found in keyspace main (keyspace has error: open /path/to/file: no such file or directory).

Related Issue(s)

Fixes #18247

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

None - error message improvement only.

AI Disclosure

Tests were written by Claude

[vitess-bot]

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[varundeepsaini]

@mattlord I think my changes aren't interfering with the tests. Can you re-run the failed checks?

[mattlord]

Thanks, @varundeepsaini ! ❤️

[varundeepsaini]

@rohit-nayak-ps could you please approve and merge this pr