[change] Return feedback if URL used for parsing is not working

[nemesifier]

In the demo system of OpenWISP, some users try to create a network topology object and insert a random URL.
This raises exceptions which are logged on the server side but not presented to the user.

We should find a way to present this information to the user, notify them, avoid logging the error on the server side.
We should also make sure that we do not bombard the user with many notifications if the error keeps on repeating.
I think we should probably modify the model to add some state which indicates whether the parsing is working fine or not.

[nemesifier]

@pandafy could we use a notification of type error here?

[pandafy]

@nemesifier this would require us to implement openwisp/openwisp-notifications#254 first

[nemesifier]

I think we can use the generic message notification here, we'll probably need to add openwisp-notifications as a dependency here.