Fixes #315 -- Set _errors when contact is removed

[oliverroick]

The error occurs when an invalid form is submitted, and the template tries to render error messages of all form fields. This fails because of a bug inside ContactsForm.full_clean(). `self._errors' is not set when the contact record is removed.

This PR fixes the issue by ensuring self._errors is set in all cases.

[seav]

I'm testing this and I can confirm that the website no longer throws an error and the Name field has a required-field error message.

However, I get weird behavior for the contacts form. Here's one:

1. Go to the add organization form
2. Add some contact data in the existing single contacts form row
3. Delete that row
4. Add a new row (the user can't make up their mind...)
5. Add some other contact data in the new row
6. Submit the data (with the Name field empty)
7. You get the expected Name is required error message, but the contacts form now contains both data that were entered.

You can try various permutations of the above steps to find more weird behavior.

[seav]

Note: I think this particular bug is fixed and can be merged. And I can just submit a new issue for the weird contacts form behavior.