[Transformer] We should throw an exception when we try to transform an invalid index

[FabienSalles]

Description

The mapping transformer uses Symfony's PropertyAccess component in order to set values.
When mappings are incorrect we need to configure the component properly if we want to see exceptions :

framework:
    property_access:
        throw_exception_on_invalid_index: true
        throw_exception_on_invalid_property_path: true

After this, we can see an exception when we have an invalid index but the second one never trigger because if the property is not writable we set the value without the property access
https://github.com/cleverage/process-bundle/blob/v3.1-dev/Transformer/MappingTransformer.php#L144

We need to remove the fallback with the array or trigger an exception before if the property does not exist