Multimedia plugin loading has explicit error reporting

The Multimedia component has explicit error reporting when loading plugins. This should probably be removed as Qt already has this (and more) debugging information available when the QT_DEBUG_PLUGINS environment variable is set to 1.

At the very least, an opt-in/out system should be utilized as some situations (such as the Simulator running on Windows) results in a bunch of plugin warnings due to the use of debug and release files.

While tracking this down, I also uncovered what appears to be an incomplete plugin-loading function. Unlike QMediaPluginLoader::load, QAudioPluginLoader::load does not clean up objects that are of an unsuitable type.