need to write a meaningful message when fails on lago start when IPv6 forwarding not configure correct

[galitf]

lago start vm-el7
@ Start specified VMs:

Start nets:

* Create network net-01: 
* Create network net-01: ERROR (in 0:00:00)

Start nets: ERROR (in 0:00:00)

@ Start specified VMs: ERROR (in 0:00:00)
Error occured, aborting
Traceback (most recent call last):
File "/home/grosenth/projects/lago/lago/cmd.py", line 969, in main
cli_plugins[args.verb].do_run(args)
File "/home/grosenth/projects/lago/lago/plugins/cli.py", line 184, in do_run
self._do_run(**vars(args))
File "/home/grosenth/projects/lago/lago/utils.py", line 505, in wrapper
return func(*args, **kwargs)
File "/home/grosenth/projects/lago/lago/utils.py", line 516, in wrapper
return func(*args, prefix=prefix, **kwargs)
File "/home/grosenth/projects/lago/lago/cmd.py", line 271, in do_start
prefix.start(vm_names=vm_names)
File "/home/grosenth/projects/lago/lago/sdk_utils.py", line 50, in wrapped
return func(*args, **kwargs)
File "/home/grosenth/projects/lago/lago/prefix.py", line 1321, in start
self.virt_env.start(vm_names=vm_names)
File "/home/grosenth/projects/lago/lago/virt.py", line 338, in start
net.start()
File "/home/grosenth/projects/lago/lago/providers/libvirt/network.py", line 115, in start
net = self.libvirt_con.networkCreateXML(self._libvirt_xml())
File "/home/grosenth/Envs/lago_venv/lib/python2.7/site-packages/libvirt.py", line 4142, in networkCreateXML
if ret is None:raise libvirtError('virNetworkCreateXML() failed', conn=self)
libvirtError: internal error: Check the host setup: enabling IPv6 forwarding with RA routes without accept_ra set to 2 is likely to cause routes loss. Interfaces to look at: enp0s31f6

message should include the problem and the solution
Solution:
Update the file:
sudo vi /etc/sysctl.conf
net.ipv6.conf.INTERFACE.accept_ra=2

where INTERFACE=enp0s31f6 in this example.

Run the command to restart the process
sudo sysctl -p

Run lago start again