Seen on master, and on 4.5.6

How to reproduce : 

1 - On calendar module, create a resource. 

2 - Book this resource for instance on 18/07 between 16 and 17h : 

3 - Create an event from 18/07 to 21/07, between 13 and 14h, and using the resource. 

4 - On week view, try to drag and drop event on 20/07 to 16h-17h, 

and choose only this event. 

5 - There is an error : 

=> Resource is available on 20/07 between 16 and 17h, error shall not be present.