75451: Show custom error message when removing last user from workflow group

dspace-api/src/main/java/org/dspace/eperson/EPersonServiceImpl.java

@@ -283,9 +283,7 @@ public class EPersonServiceImpl extends DSpaceObjectServiceImpl<EPerson> impleme
         for (Group group: workFlowGroups) {
             List<EPerson> ePeople = groupService.allMembers(context, group);
             if (ePeople.size() == 1 && ePeople.contains(ePerson)) {
-                throw new IllegalStateException(
+                throw new EmptyWorkflowGroupException(ePerson.getID(), group.getID());
-                    "Refused to delete user " + ePerson.getID() + " because it the only member of the workflow group"
-                    + group.getID() + ". Delete the tasks and group first if you want to remove this user.");
             }
         }
         // check for presence of eperson in tables that

dspace-api/src/main/java/org/dspace/eperson/EmptyWorkflowGroupException.java

@@ -0,0 +1,24 @@
+package org.dspace.eperson;
+
+import java.util.UUID;
+
+/**
+ * <p>This exception class is used to distinguish the following condition:
+ * EPerson cannot be deleted because that would lead to one (or more)
+ * workflow groups being empty.</p>
+ *
+ * <p>The message of this exception can be disclosed in the REST response to
+ * provide more granular feedback to the user.</p>
+ *
+ * @author Bruno Roemers (bruno.roemers at atmire.com)
+ */
+public class EmptyWorkflowGroupException extends IllegalStateException {
+
+    public EmptyWorkflowGroupException(UUID ePersonId, UUID groupId) {
+        super(String.format(
+            "Refused to delete user %s because it is the only member of the workflow group %s. " +
+            "Delete the tasks and group first if you want to remove this user.", ePersonId, groupId
+        ));
+    }
+
+}

dspace-server-webapp/src/main/java/org/dspace/app/rest/exception/DSpaceApiExceptionControllerAdvice.java

@@ -98,6 +98,14 @@ public class DSpaceApiExceptionControllerAdvice extends ResponseEntityExceptionH
                 HttpStatus.UNPROCESSABLE_ENTITY.value());
     }
 
+    @ExceptionHandler(RESTEmptyWorkflowGroupException.class)
+    protected void handleEmptyWorkflowGroupException(HttpServletRequest request, HttpServletResponse response,
+                                                     Exception ex) throws IOException {
+        sendErrorResponse(
+            request, response, null, ex.getLocalizedMessage(), HttpStatus.UNPROCESSABLE_ENTITY.value()
+        );
+    }
+
     @ExceptionHandler(QueryMethodParameterConversionException.class)
     protected void ParameterConversionException(HttpServletRequest request, HttpServletResponse response, Exception ex)
         throws IOException {

dspace-server-webapp/src/main/java/org/dspace/app/rest/exception/RESTEmptyWorkflowGroupException.java

@@ -0,0 +1,29 @@
+/**
+ * The contents of this file are subject to the license and copyright
+ * detailed in the LICENSE and NOTICE files at the root of the source
+ * tree and available online at
+ *
+ * http://www.dspace.org/license/
+ */
+package org.dspace.app.rest.exception;
+
+/**
+ * <p>Extend {@link UnprocessableEntityException} to provide a specific error message
+ * in the REST response. The error message is added to the response in
+ * {@link DSpaceApiExceptionControllerAdvice#handleEmptyWorkflowGroupException}.</p>
+ *
+ * <p>Note there is a similarly named error in the DSpace API module.</p>
+ *
+ * @author Bruno Roemers (bruno.roemers at atmire.com)
+ */
+public class RESTEmptyWorkflowGroupException extends UnprocessableEntityException {
+
+    public RESTEmptyWorkflowGroupException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+    public RESTEmptyWorkflowGroupException(String message) {
+        super(message);
+    }
+
+}

dspace-server-webapp/src/main/java/org/dspace/app/rest/repository/EPersonRestRepository.java

@@ -22,6 +22,7 @@ import org.dspace.app.rest.Parameter;
 import org.dspace.app.rest.SearchRestMethod;
 import org.dspace.app.rest.authorization.AuthorizationFeatureService;
 import org.dspace.app.rest.exception.DSpaceBadRequestException;
+import org.dspace.app.rest.exception.RESTEmptyWorkflowGroupException;
 import org.dspace.app.rest.exception.UnprocessableEntityException;
 import org.dspace.app.rest.model.EPersonRest;
 import org.dspace.app.rest.model.MetadataRest;
@@ -34,6 +35,7 @@ import org.dspace.authorize.service.AuthorizeService;
 import org.dspace.content.service.SiteService;
 import org.dspace.core.Context;
 import org.dspace.eperson.EPerson;
+import org.dspace.eperson.EmptyWorkflowGroupException;
 import org.dspace.eperson.RegistrationData;
 import org.dspace.eperson.service.AccountService;
 import org.dspace.eperson.service.EPersonService;
@@ -313,6 +315,8 @@ public class EPersonRestRepository extends DSpaceObjectRestRepository<EPerson, E
             es.delete(context, eperson);
         } catch (SQLException | IOException e) {
             throw new RuntimeException(e.getMessage(), e);
+        } catch (EmptyWorkflowGroupException e) {
+            throw new RESTEmptyWorkflowGroupException(e.getLocalizedMessage(), e);
         } catch (IllegalStateException e) {
             throw new UnprocessableEntityException(e.getMessage(), e);
         }