Improve "VM limit exhausted" message

[fmoc]

Right now, this is the log output when trying to build a new VM with Packer when two are already running:

% packer build [...]
tart-cli.tart: output will be in this color.

==> tart-cli.tart: Cloning virtual machine...
==> tart-cli.tart: Updating virtual machine resources...
==> tart-cli.tart: Inspecting machine disk image...
==> tart-cli.tart: Getting partition table...
==> tart-cli.tart: Starting the virtual machine...
==> tart-cli.tart: Waiting for the VNC server credentials from Tart...
==> tart-cli.tart: Retrieved VNC credentials, connecting...
==> tart-cli.tart: Failed to connect to the Tart's VNC server: dial tcp 127.0.0.1:60993: connect: connection refused
==> tart-cli.tart: Waiting for the tart process to exit...
Build 'tart-cli.tart' errored after 1 second 61 milliseconds: Failed to connect to the Tart's VNC server: dial tcp 127.0.0.1:60993: connect: connection refused

It took me a while to figure out it's the 2 VMs limit which is responsible for this log output. Is there a way to catch the limit earlier and log this appropriately?