MC-107: Add an option to force attribute set removal

Bug fix: [no]
Feature addition: [yes]
Backwards compatibility break: [no]
Phpspec specification passes: [no]
Checkstyle issues: [no]
ChangeLog updated: [no]
Fixes the following issue: https://github.com/akeneo/MagentoConnectorBundle/issues/281
SOAP_FAULT would be better than SOAP_ACTION. And something along the lines of SOAP_FAULT_PRODUCTS_IN_SET would be clearer. The conveyed meaning of SOAP_ACTION_UNABLE_REMOVE_ATTRIBUTE_SET is more about the consequences than about the reason.