Did you click on the "More information about this error?" because you will find the answer there. It says this...
A broken plugin, such as a module, has been detected. In order for the upgrade to proceed, the broken plugin should be deleted.
In the case of a Theme, then it is more than likely there is a file that has not been renamed correctly, or the theme is missing a vital component like a version.php, or that the version.php is not correct in some small detail, usually the plugin version.
For more information read Themes 2.0
However, in your case the fact you are calling your theme Magpie is a probelm too, not the name but the fact you have used a Capital letter. In Moodle you cannot use Caps...end of story!