Child "bank" does not exist.

Given HTML5 form validation is disabled
And the user does not enter a bank
When the user submits the form
Then...

ChoosePaymentMethodType::validate() is called
PluginControllerInterface::checkPaymentInstruction() returns dataErrors
ChoosePaymentMethodType::applyErrorsToForm() fails because
$path equals "bank"

The result is that Symfony Form could not find a child "bank" in the root form and throws an exception.

This pull request fixes that bug.