Improved debugging

Use ToString of the expression as message, this allows usage of
PartialEvaluationExceptionExpression ToString output

Should help when debugging the following stacktrace:

System.NotSupportedException: PartialEvaluationExceptionExpression
   at NHibernate.Linq.Visitors.HqlGeneratorExpressionTreeVisitor.VisitExpression(Expression expression)
   at NHibernate.Linq.Visitors.HqlGeneratorExpressionTreeVisitor.VisitBinaryExpression(BinaryExpression expression)
...