Fix MappingMatcher in case of an exception in LinqMatcher. (#322)

* Fix MappingMatcher in case of an exception in LinqMatcher.

* update unit-tests

src/WireMock.Net/Matchers/XPathMatcher.cs

@@ -59,6 +59,7 @@ namespace WireMock.Matchers
                 catch (Exception)
                 {
                     // just ignore exception
+                    // TODO add logging?
                 }
             }