error on adding a new election for a candidate but marking them as "don't know"

[mhl]

We got some email errors like this:

[...]/candidates/models/popolo_extra.py", line 100, in update_person_from_form
    post=post,
UnboundLocalError: local variable 'post' referenced before assignment

I can reproduce this on Democracy Club's master branch by:

• Go to a person edit page
• Clicking "Add a new election for this person"
• Select an election (e.g. London Mayoral)
• Don't change the "Standing in London Mayoral:" drop-down; i.e. you should leave it at "Don't know"
• Adding a source and submitting the change.