Backend crash on Not implemented Error: SAVEPOINT is not supported in DuckDB

### What happens?

With `duckdb.force_execution` set to TRUE, this query crashes user backend:
```sql
DO $$
DECLARE
objtype text;
BEGIN
FOR objtype IN VALUES ('toast table'), ('index column'), ('sequence column'),
('toast table column'), ('view column'), ('materialized view column')
LOOP
BEGIN
PERFORM pg_get_object_address(objtype, '{one}', '{}');
EXCEPTION WHEN invalid_parameter_value THEN
RAISE WARNING 'error for %: %', objtype, sqlerrm;
END;
END LOOP;
END;
$$;
```

Result:
```sql
psql:orig.sql:18: FATAL: (PGDuckDB/DuckdbSubXactCallback_Cpp) Not implemented Error: SAVEPOINT is not supported in DuckDB
CONTEXT: PL/pgSQL function inline_code_block line 8 during statement block entry
psql:orig.sql:18: server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
psql:orig.sql:18: error: connection to server was lost
```

This seems to be happening only in PostgreSQL 17+ versions due to [this](https://github.com/postgres/postgres/blob/0075d78947e3800c5a807f48fd901f16db91101b/src/backend/access/transam/xact.c#L4667) change that treats any error in subtransaction as fatal.

In PostgreSQL 16, for instance, we won't get backend crash:
```
ERROR: (PGDuckDB/DuckdbSubXactCallback_Cpp) Not implemented Error: SAVEPOINT is not supported in DuckDB
CONTEXT: PL/pgSQL function inline_code_block line 8 during statement block entry
```

This may look as expected behavior in PostgreSQL 17+, the question is if we really want to get backend crashed in these situations or we may find some way to handle unsupported SAVEPOINTS without causing backend crash.

### To Reproduce

1. Start a fresh server with pg_duckdb and `duckdb.force_execution` set to TRUE
2. Run this query:
```sql
DO $$
DECLARE
objtype text;
BEGIN
FOR objtype IN VALUES ('toast table'), ('index column'), ('sequence column'),
('toast table column'), ('view column'), ('materialized view column')
LOOP
BEGIN
PERFORM pg_get_object_address(objtype, '{one}', '{}');
EXCEPTION WHEN invalid_parameter_value THEN
RAISE WARNING 'error for %: %', objtype, sqlerrm;
END;
END LOOP;
END;
$$;
```
3. Get a backend crash

### OS:

Linux

### pg_duckdb Version (if built from source use commit hash):

cbc28da57aa943066af41ffa520c631a39046cf1

### Postgres Version (if built from source use commit hash):

17

### Hardware:

_No response_

### Full Name:

Jora Babayan

### Affiliation:

Postgres Professional

### What is the latest build you tested with? If possible, we recommend testing with the latest nightly build.

I have tested with a source build

### Did you include all relevant data sets for reproducing the issue?

No - Other reason (please specify in the issue body)

### Did you include all code required to reproduce the issue?

- [x] Yes, I have

### Did you include all relevant configuration (e.g., CPU architecture, Linux distribution) to reproduce the issue?

- [x] Yes, I have