Unavailability of task export when locked because of "Excel external editing" must be communicated to the user in the right way

If the task is locked because auf Excel export currently an error message appears in the new window, that opens for the download. The error message is not seen by the user, because the browser shows a "json syntax error".

This should be improved by

• Disabeling the export button in the export option of the task (but it should still be there)
• Visualizing in the export button, why the export is disabled