Fix type errors in cli.py and db.py

- Add type annotation for Database.conn to fix context manager errors
- Convert exception objects to str() when raising ClickException
- Handle None return from find_spatialite() with proper error message

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sqlite_utils/db.py

@@ -328,6 +328,7 @@ class Database:
 
     _counts_table_name = "_counts"
     use_counts_table = False
+    conn: sqlite3.Connection
 
     def __init__(
         self,