Improve exception for empty configuration parameter javax.ws.rs.Application

RESTEasy can be configured through several configuration options in WAR application deployment file WEB-INF/web.xml. One of them (defined by JAX-RS standard) is javax.ws.rs.Application. When this parameter is empty, exception "java.lang.StringIndexOutOfBoundsException: String index out of range: 0" is raised. Would be nice to throw exception with detailed message, eg.

throw new ApplicationException("The implementation of javax.ws.rs.core.Application must be specified.", null)