Fix missing wrap_db_error for Session.execute() method

We should add wrap_db_error for Session.execute() method, because
Query.update() method will call Session.execute() and it is not inside
Session.flush(), so exceptions from Query.update() wouldn't be wrapped.

Add test for Session.flush() wrapper
Add test for Session.execute() wrapper

Fixes bug 1107890

Change-Id: I96894e502f9f279999b61a1e66469e34a07a022d