Web server: don't return stack trace in error document

For example, when a specific job that does not exist is queried with `/jobs/unexisting-job`, you get a 404 (this is correct) and a `error` document that looks something like this:

```xml

Job not found
java.lang.Throwable: Job not found
at org.daisy.pipeline.webservice.impl.GenericResource.getErrorRepresentation(GenericResource.java:36)
at org.daisy.pipeline.webservice.impl.JobResource.getResource(JobResource.java:78)
at sun.reflect.GeneratedMethodAccessor30.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.restlet.resource.ServerResource.doHandle(ServerResource.java:506)
at org.restlet.resource.ServerResource.get(ServerResource.java:707)
at org.restlet.resource.ServerResource.doHandle(ServerResource.java:589)
at org.restlet.resource.ServerResource.doNegotiatedHandle(ServerResource.java:649)
at org.restlet.resource.ServerResource.doConditionalHandle(ServerResource.java:348)
at org.restlet.resource.ServerResource.handle(ServerResource.java:951)
at org.restlet.resource.Finder.handle(Finder.java:246)
at org.restlet.routing.Filter.doHandle(Filter.java:159)
at org.restlet.routing.Filter.handle(Filter.java:206)
at org.restlet.routing.Router.doHandle(Router.java:431)
at org.restlet.routing.Router.handle(Router.java:648)
...

```

The "Job not found" message is helpful, but the stack trace is just implementation detail.