ReadResourceHandler should deal with resources that disappear in the middle of the request

ReadResourceHandler works by adding a number of steps and assembling the results. Steps are added to read each attribute and to read children (if recursive=true).

With runtime resources, the resource associated with each added step may have disappeared between when the code decided to do the read and when the step executes. We should detect and handle this, treating it as if the resource in question never existed.

See discussion on https://github.com/wildfly/wildfly/pull/6607.