Improve ShouldThrow by having it throw MatchException.

src/Fixie.Tests/Assertions/AssertionExtensions.cs

@@ -84,7 +84,7 @@ public static void ShouldBeEmpty<T>(this IEnumerable<T> collection)
                 return (TException)actual;
             }
 
-            throw new AssertException("Expected an exception to be thrown.");
+            throw new MatchException(typeof(TException).FullName, "No exception was thrown.");
         }
 
         public static void ShouldBeGreaterThan<T>(this T actual, T minimum) where T: IComparable<T>