URL Rewriter: Explicitly warn when the path to the rewrite documents cannot be found

The only place where is checked if the rewriteRulesLocation exists is in the Rewritingmanager. It is checked only once, which is good, because if it really is not there, it should not be rechecked over and over again.

But in case of a race condition at startup (rules are attempted to be loaded but the repository is not yet available) the rules are not loaded until a rule document node is changed in the repository. A more explicit check of the path and warning from the RewritingManager may help distinguishing this situation.