You need to also add it to the message dependencies of the ext.uploadWizard ResourceLoader module - backend logic can look up messages on the spot, frontend logic cannot.
Probably something like
$wgExtensionFunctions[] = function () {
    $wgResourceModules['ext.uploadWizard']['messages'][] = 'mwe-upwiz-license-c';
would work. Although UploadWizard should do this itself if it exposes its messages to be configurable, so I'd say this is a bug.