Generic error message in the new view when creating a sub task

Issue Summary

If a project has a default assignee but the default assignee does not have the assignable user permission, when creating a subtask through the new issue view, users would be prompted a generic error message stating that

We couldn't create the subtask

Check your connection, then give it another try

Steps to Reproduce

1. Set default assignee on Project Settings details tab
2. Make sure default assignee does not have the assignable user permission
3. Go to any task and make sure you're in the new issue view
4. Create sub task

Expected Results

Just like the old view, error message that states "The default assignee does NOT have ASSIGNABLE permission OR Unassigned issues are turned off."

Actual Results

Generic error message is prompted

Workaround

Create sub task in the old view to see the error message or run the developer console to see the error. The error message is visible in Console log:

{"errorMessages":[],"errors":{"assignee":"The default assignee does NOT have ASSIGNABLE permission OR Unassigned issues are turned off."}}