improve the error diagnostics.

during the databinding, failure to find a descriptor is an error.

core/src/main/resources/lib/form/repeatable.jelly

@@ -113,7 +113,7 @@ THE SOFTWARE.
       <j:set var="items" value="${instance[name] ?: attrs.default}"/>
 
       <!-- and expose update descriptor to the body of this tag -->
-      <j:set var="descriptor" value="${descriptor.getPropertyType(instance,field).itemTypeDescriptor}" />
+      <j:set var="descriptor" value="${descriptor.getPropertyType(instance,field).itemTypeDescriptorOrDie}" />
     </j:when>
     <j:otherwise>
       <j:set var="name" value="${attrs.name?:attrs.var}"/>