### Steps to reproduce

1. Create an invalid user mention (e.g. a deleted Begin of the insertionuser `[~UserDoesNotExist]` )End of the insertion Begin of the deletion user)End of the deletion in a Jira issue description field

2. Login as a user with permissions to import Jira

3. Import that Jira issue


### What is the buggy behavior?

* Import stops with a `Jira API returned error status 404` message


### What is the expected behavior?

1. The non-existing user is skipped

2. Begin of the insertion The mention in the OpenProject description is not linked but inserted as plain text @UserDoesNotExist instead

3.End of the insertion The import continues


### Environment information

**OpenProject installation type**

* All


**OpenProject version**

**Browser**

* All


**Operating System**

* All


**Language**

* All