Fix database dump silent failures on cursor close errors

[no-itsbackpack]

Summary

Fixes a regression introduced in commit 4d3dc85 where pscale database dump could finish successfully after a partial dump when the MySQL stream failed mid-query.

Root cause

When refactoring dumpTable for multiple output formats, the explicit cursor.Close() error check was replaced with defer cursor.Close(). In go-mysqlstack, stream errors (e.g. unexpected EOF, closed network connection) are often only returned from Close() after Next() stops, so those errors were swallowed and dumpTable returned success.

Changes

• Restore an explicit cursor.Close() check after finishing the row loop and writer flush.
• Propagate dumpTable errors from errgroup workers so Run() fails and pscale database dump exits non-zero.
• Add tests for cursor close error propagation and dump failure exit behavior.

[cursor]

Risk: medium. Left a non-blocking comment; no reviewers assigned. Cursor Bugbot reported an unresolved high-severity cursor leak on early exits, so this PR is not approved and needs human review.

  

_{Sent by Cursor Approval Agent: cli-reviewer}

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

There are 3 total unresolved issues (including 2 from previous reviews).

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