Fix bug with response code && Add the 50002 error message

Paymill response code is JMS reason code (see [paymill
tests](https://github.com/paymill/paymill-php/blob/2eefd630f014c2ece9857bf0ba3fd920deb52208/tests/unit/Paymill/Services/ResponseHandlerTest.php#L621-L653)). The ```status``` is JMS response code.

This lead to a standard message ```an error has occured``` or something
like that  but no detail.

The financial transaction was missing when the transaction was pending
Merge pull request #13 from jdeniau/fix-exception-handling

Fix bug with response code && Add the 50002 error message
Merge branch 'add-offer-subscription' of github.com:jdeniau/paymill-bundle into add-offer-subscription