Better handling when rack becomes full

[timalces]

Rack height is an arbitrary value relevant solely in visualiser. It is therefore possible for a stack to be created that exceeds this height, or when new device types are implemented (e.g. volumes are included when they weren't before), or if in the future users are given the ability to create new devices for existing stacks.

Currently this results in the middleware being unable to create the device in visualiser and it erroring - we should implement a better way of handling this. We may also want to consider how to handle having a very tall rack, as when shown in the IRV this will effectively zoom out all racks & devices present.