(DNN-3890) Add failing tests for misconfigured Extension URL Provider

If an extension URL provider is registered with a type that doesn't exist, the
whole site dies.  It should, instead, ignore misconfigured providers.  This
adds tests to demonstrate the issue.
(DNN-3890) When Extension URL Provider is misconfigured, ignore it

If an extension URL provider is registered with a type that doesn't exist, the
whole site dies.  It should, instead, ignore misconfigured providers.  This
commit ignores providers that cannot be instantiated.
Merge branch 'url-provider' of https://github.com/bdukes/Dnn.Platform into bdukes-url-provider
DNN-3890 >Extension URL Provider with Type that cannot be found causes whole site to die
Merge branch 'bdukes-url-provider' into development