Remove slug from the "package not found" error message.

A user reported that the slug could be used for content injection attacks. While it's impossible to inject HTML/CSS/JS code, even plain text can  theoretically be abused for social engineering purposes. 

I'd like to allow relatively free-form slugs (basically, any valid file name can be a slug), so there doesn't seem to be a way to eliminate this kind of risk other than just not showing the slug.

includes/Wpup/UpdateServer.php

@@ -153,7 +153,7 @@ protected function validateRequest($request) {
 			$this->exitWithError('You must specify a package slug.', 400);
 		}
 		if ( $request->package === null ) {
-			$this->exitWithError(sprintf('Package "%s" not found', htmlentities($request->slug)), 404);
+			$this->exitWithError('Package not found', 404);
 		}
 	}