Display a Better Error Message when a Project Admin Attempts to Change the Default Assignee

Problem Definition

When a Project Admin who does not have JIRA Administrator Global Permissions attempts to change the Default Assignee for their Project, it generates an error stating:

You must have global administrator rights in order to modify the project key.

Per Defining a Project - A note about Project Administrators, Project Admins do not have the ability to change the Default Assignee.

Suggested Solution

Remove the option to change the Default Assignee for Project Admin only users
or,
Display a useful message indicating that the Project Admin cannot change the Default Assignee because they lack JIRA Administrator Global Permissions.
Ex:

You must have global administrator rights in order to modify the default assignee.

Workaround

If you add the user to the administrators group, they will be able to edit the Project Lead. Or if that is not possible, another project administrator in the administrators group can make the change.