Improve exception message for missing properties

I had some old code (NHibernate 1.0.2) that initialized by using the following:

configurationProperties.Add("hibernate.dialect", "NHibernate.Dialect.MsSqlCeDialect");

I'm just migrating my project to 2.1.0Alpha2 and I got the following very confusing exception:

System.InvalidOperationException: Could not find the dialect in the configuration
at NHibernate.Dialect.Dialect.GetDialect(IDictionary`2 props)
at NHibernate.Cfg.SettingsFactory.BuildSettings(IDictionary`2 properties)
at NHibernate.Cfg.Configuration.BuildSettings()
at NHibernate.Cfg.Configuration.BuildSessionFactory()

This could be made clearer. As it stands "could not find the dialect in the configuration" could mean one of two things:
1. The dialect named in the configuration could not be found (because of missing dependencies, or because it's not a recognised dialect)
2. The configuration doesn't contain a setting for the dialect.

I spent a fair bit time assuming that it was the first option (because this is a new box, and I wasn't sure if everything was installed correctly), when it was actually the second meaning.

I suggest changing the text to something like: "The configuration does not define a property named 'dialect'". Then I'd have known that the syntax for properties had changed.