Importing duplicate cable raises error "Cable has no termination_a_type"

[sulisu]

Environment

• Python version: 3.5.3
• NetBox version: 2.6.7

Steps to Reproduce

1. Import a cable between two interface, which already exist in netbox
   Example Data

side_a_device,side_a_type,side_a_name,side_b_device,side_b_type,side_b_name,type,status
ZBP06U0708,interface,port3,ZBP06S6520X1,interface,XGE1/0/2,Multimode Fiber (OM3),Planned

It will sucess at first time when there is no same cable definition in netbox

2. Import the above csv data again

<class 'dcim.models.Cable.termination_a_type.RelatedObjectDoesNotExist'>

Cable has no termination_a_type.

Expected Behavior

A more clear error definition, which indicate there is a duplicate, will be very helpful.

Observed Behavior

An error message seems not relating to the real problem.