libvirt-machine: More specific error on start failing
machine: Better message on failure to start

We were showing "Connection to MACHINE_NAME failed" even when error had
nothing to do with connection. With this patch, we say "Failed to start
MACHINE_NAME" and show a warning on console with underlying error message.
machine: Warning on failing to connect to display

Instead of outputting to debug log (which is suppressed by default),
output error on warning log. Also, improve the message a bit.