set_interface() always POSTed to vlans-ports, treating every membership as
new. Moving a port that already carries the VLAN from untagged to tagged is
not a create, though, and the switch says so:
POST vlans-ports {"vlan_id":10,"port_id":"10","port_mode":"POM_TAGGED_STATIC"}
-> 400 {"message":"Association exists"}
Only 409 was handled as "already there"; v7 firmware answers 400. The
membership is its own resource named {vlan_id}-{port_id} and takes a PUT:
PUT vlans-ports/10-10 -> 200
So the exact case anyone hits first failed outright — a port holding VLAN 10
untagged alongside 30/40/50 tagged, with VLAN 10 to become tagged too.
The response body is now carried into both error messages. The ports PUT just
above already did this; here it was dropped, so "Association exists" — which
names the cause outright — never reached the caller. The message read
"vlans-ports POST HTTP 400" and nothing more.
Verified against a 2530-24G-PoEP on REST v7: set_interface("10", trunk
vlan 30), where that membership already exists, now completes and leaves the
port exactly as it was.