500 when saving a Route with misconfigured parent / slug

[meshy]

When a route is saved in the admin with an invalid combination of parent and slug, a 500 is raised. This needs proper validation.

	parent	no parent
slug	branch / leaf node	invalid
no slug	invalid	root node

Perhaps the message could be clearer too?

[meshy]

It looks as though the validation that exists here can be moved from save to clean and should raise ValidationError rather than ValueError.

[meshy]

Closing in favour of #75