Handle a PR already existing for post submission

[andy-wojciechowski]

Currently if you try and submit a post which uses a GitHub branch that is already in PR an error is thrown. We should add some sort of handling for that case.

Steps to Reproduce:

1. Submit a post with the editor with the title "Test Post"
2. Try to submit another post with the title "Test Post"

Expected Result:
An error should be returned to the user informing them a post with that given title has already been submitted.

Actual Result:
An Octokit::UnprocessableEntity error is thrown.

[GFLEMING133]

I think the correct answer here would be to return an error to the user saying a post already exists with that title. The error message should inform the user that if they need to, they can edit the existing post (the post that already exists with the submitted title), and if they don't need to, they should rename their current post.

[andy-wojciechowski]

Ok, I've updated the expected result to say that an error should be returned.