NPE in URLDeploymentScanner

The URLDeploymentScanner throws NullPointerExceptions if it is not configured correctly.
e.g. no URLComparator or Filter attribute

Since this xml config is an adminstrative interface,
we cannot expect administrators to understand what this means.

The implementation should either:
1) Provide default configurations for these mandatory attributes
2) Check the configuration in createService() and throw human readable deployment exceptions
if mandatory configuration is missing.