Adds some more error checking to PhysicalFileSystem ctor. Fixing issues with Script/Style repo tests.

src/Umbraco.Core/IO/PhysicalFileSystem.cs

@@ -18,7 +18,11 @@ namespace Umbraco.Core.IO
 
         public PhysicalFileSystem(string virtualRoot)
         {
-            RootPath = IOHelper.MapPath(virtualRoot);
+	        if (virtualRoot == null) throw new ArgumentNullException("virtualRoot");
+			if (!virtualRoot.StartsWith("~/"))
+				throw new ArgumentException("The virtualRoot argument must be a virtual path and start with '~/'");
+
+	        RootPath = IOHelper.MapPath(virtualRoot);
             _rootUrl = IOHelper.ResolveUrl(virtualRoot);
         }
 
@@ -30,6 +34,9 @@ namespace Umbraco.Core.IO
             if (string.IsNullOrEmpty(rootUrl))
                 throw new ArgumentException("The argument 'rootUrl' cannot be null or empty.");
 
+			if (rootPath.StartsWith("~/"))
+				throw new ArgumentException("The rootPath argument cannot be a virtual path and cannot start with '~/'");
+
             RootPath = rootPath;
             _rootUrl = rootUrl;
         }