Restful viewer doesn't return a JSON representation when hits a 500 internally (instead getting an HTML page)

originally raised on the dev mailing list by Erik de Hair...

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I've reproduce the issue, by introducing a deliberate bug in the todo app, changing one of the queries in ToDoItem:

@javax.jdo.annotations.Query(
name = "findByOwnedByAndCompleteIsFalse", language = "JDOQL",
value = "SELECT "
+ "FROM dom.todo.ToDoItem "
+ "WHERE ownedBy == :ownedBy "
+ " && complete = false"),

where "complete == false" has been changed to "complete = false".

In the Wicket viewer - as hoped - I got the error page with a big nasty stacktrace that included:

javax.jdo.JDOUserException
Invalid operator "=". Did you mean to use "=="?
org.datanucleus.api.jdo.NucleusJDOHelper#getJDOExceptionForNucleusException(NucleusJDOHelper.java:549)
org.datanucleus.api.jdo.JDOQuery#executeWithMap(JDOQuery.java:351)
org.apache.isis.objectstore.jdo.datanucleus.persistence.queries.PersistenceQueryFindUsingApplibQueryProc

In the Restful viewer, when I invoked http://localhost:8080/restful/services/ToDoItems/actions/notYetComplete/invoke I got something an HTML error:

HTTP ERROR 500

Problem accessing /restful/services/ToDoItems/actions/notYetComplete/invoke. Reason:

javax.jdo.JDOUserException: Invalid operator "=". Did you mean to use "=="?
NestedThrowables:
org.datanucleus.store.query.QueryCompilerSyntaxException: Invalid operator "=". Did you mean to use "=="?
Caused by:

org.apache.isis.core.commons.exceptions.IsisException: javax.jdo.JDOUserException: Invalid operator "=". Did you mean to use "=="?
NestedThrowables:
org.datanucleus.store.query.QueryCompilerSyntaxException: Invalid operator "=". Did you mean to use "=="?
at org.apache.isis.core.runtime.system.transaction.IsisTransactionManager.executeWithinTransaction(IsisTransactionManager.java:217)
at org.apache.isis.core.runtime.transaction.facets.ActionInvocationFacetWrapTransaction.invoke(ActionInvocationFacetWrapTransaction.java:54)
at org.apache.isis.core.metamodel.specloader.specimpl.ObjectActionImpl.execute(ObjectActionImpl.java:367)
at org.apache.isis.viewer.restfulobjects.server.resources.DomainResourceHelper.invokeActionUsingAdapters(DomainResourceHelper.java:306)
at org.apache.isis.viewer.restfulobjects.server.resources.DomainResourceHelper.invokeActionQueryOnly(DomainResourceHelper.java:251)
at org.apache.isis.viewer.restfulobjects.server.resources.DomainServiceResourceServerside.invokeActionQueryOnly(DomainServiceResourceServerside.java:165)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

~~~~

So, I think the issue is that the Restful viewer, although it's throwing a 500 response error as expected, is also returning HTML rather than JSON error page.