Add root exception to new exception

This is needed to get an idea why the context could not be created.

scm-dao-xml/src/main/java/sonia/scm/store/TypedStoreContext.java

@@ -48,7 +48,7 @@ final class TypedStoreContext<T> {
       JAXBContext jaxbContext = JAXBContext.newInstance(parameters.getType());
       return new TypedStoreContext<>(jaxbContext, parameters);
     } catch (JAXBException e) {
-      throw new StoreException("failed to create context for store");
+      throw new StoreException("failed to create context for store", e);
     }
   }