You'll find the exhaustion message in the logs. The user will just get a message saying "disconnected" when the webtop page is being shown so no they don't get specifically shown what the error is. It's actually frustrating for the end user to get a non-descript message like that. I haven't seen in in the documentation but have this issue currently so I'm aware of what the user sees.
This is the APM log message
IPv4 Allocation failure: leasepool /Common/Default_Pool is out of addresses
Not much of an answer but there you go ... it's a simple question :)