SQLAlchemy: Improve error message when using `schema` query parameter twice

### Problem

When approaching the CrateDB SQLAlchemy dialect with an erroneous connection string including the `schema` query parameter twice, like `crate://localhost/?schema=foo&schema=bar`, it fails using this exception message:
```python
(crate.client.exceptions.ProgrammingError) expected string or bytes-like object, got 'tuple'
```

The error message may be improved.

### References

- https://github.com/crate/cratedb-examples/pull/174#pullrequestreview-1761390511