Improved debugging output

Use ToString of the expression as message, this allows usage of
PartialEvaluationExceptionExpression ToString output
Merge pull request #208 from janv8000/master

Improved debugging
NH3392 - Unit test and fix
Merge pull request #239 from gliljas/NH-3392

NH3392 - Unit test and fix
NH3392/Fixture.cs: Minor corrections.