Add option to stop machine on error warning

[Emerson Prado]

Sometimes hardware issues cause VB to show a warning when opening a VM. This warning could give the user the option not to start the VM. This would be useful for allowing the error to be corrected before indeed running the VM. I sometimes get errors due to an issue in a serial peripheral I have (which doesn't seem to have a fix). When I open the VM, I get a warning - then I have to wait for the guest OS to load, then shut it down. Instead, I'd like to tell VB not to open the VM, correct the error condition, and then open the VM correctly.