Using more adapted when no connection string is found

--HG--
branch : dev

src/Orchard/Data/Providers/SqlServerDataServicesProvider.cs

@@ -18,7 +18,7 @@ namespace Orchard.Data.Providers {
         public override IPersistenceConfigurer GetPersistenceConfigurer(bool createDatabase) {
             var persistence = MsSqlConfiguration.MsSql2008;
             if (string.IsNullOrEmpty(_connectionString)) {
-                throw new NotImplementedException();
+                throw new ArgumentException("The connection string is empty");
             }
             persistence = persistence.ConnectionString(_connectionString);
             return persistence;