Fix buffer search invalid regexp indicator (#7795)

This PR fixes the issue with invalid regexp highlighting (red border)
when switching to the simple text searching mode (#7658).

Implementation details:

- `update_matches()` always relied on the caller to reset the
`query_contains_error` flag, which wasn't always the case. Now, it
resets the flag itself.

Release Notes:

- Fix issue with switching between invalid regexp and simple text buffer
search (#7658).

How it works now:


https://github.com/zed-industries/zed/assets/2101250/ac868a5d-5e2f-49a0-90fc-00e62a1d5ee8