Do not clear returned results on query cancel

## What did you do?

- Go to "SQL Query"
- Execute long running query which returns many rows
- Once first rows starting to be displayed (returned) - press "Cancel"

## What did you expect to happen?

- I expect the output that already returned to stay visible in results

## What actually happened?

- All returned rows are wiped and "cancel" message is displayed

## What software versions are you using?

Postico version:
Version 1.5.16 (7440)

macOS version:
10.15.7

PostgreSQL version:
PostgreSQL 8.0.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.4.2 20041017 (Red Hat 3.4.2-6.fc3), Redshift 1.0.21556

Start at the SQL Query entry point and trace how Cancel handles rows that have already been returned. Verify the behavior with a long-running query that begins displaying rows; done means cancellation stops further work without removing visible results, while the cancellation message remains available.

Written by the indexing model from the issue text.