changed deprecated FormException in favor of InvalidConfigurationException
only valid variants should be dispatched
made variant generator flash message translatable
Merge pull request #1136 from akovalyov/fix/generator-events-dispatching

[VariableProductBundle] Only valid variants should be dispatched
Merge pull request #1138 from akovalyov/fix/variant-generator-flash-translations

Variant generator flash message should be translatable