Fix return type of Connection::execute

[benjaminfayaz]

No description provided.

[t-gebauer]

Related: #114

[thePanz]

thanks @benjaminfayaz for this fix!
According to Connection::prepare($statement) the return could also be void in case of an Doctrine_Adapter_Exception or PDOException.

That should be added too IMO, WDYT?

The same applies for Connection::execute(), as its last line $this->rethrowException(...) might not re-throw the exception and return.