introduce custom UsageError

This is a exception to be thrown when the error occures because
of bad user data. We don't want to print a full stack trace in
these cases but just tell the user what went wrong.

test/python/test_db_connection.py

@@ -28,5 +28,5 @@ def test_cursor_scalar(db, temp_db_cursor):
 
 def test_cursor_scalar_many_rows(db):
     with db.cursor() as cur:
-        with pytest.raises(ValueError):
+        with pytest.raises(RuntimeError):
             cur.scalar('SELECT * FROM pg_tables')