Properly display errors when posting new channel forms

Currently we raise a ``ValueError`` and return a 500 response:

```python
Traceback (most recent call last):

  File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 115, in get_response
    response = callback(request, *callback_args, **callback_kwargs)

  File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/django/contrib/auth/decorators.py", line 25, in _wrapped_view
    return view_func(request, *args, **kwargs)

  File "/var/praekelt/vumi-go/go/channel/views.py", line 89, in new_channel
    raise ValueError(repr('Error: %s' % (form.errors,)))

ValueError: 'Error: <ul class="errorlist"><li>country<ul class="errorlist"><li>This field is required.</li></ul></li><li>channel<ul class="errorlist"><li>This field is required.</li></ul></li></ul>'
```