Doesn't show error when shortening URL with keyword that already exists

[ghost]

When someone shortens a URL and specifies a custom keyword that is already taken, the theme will not show an error but instead just show the short URL box to be empty on the results page.

[tomslominski]

I believe this might be related to YOURLS/YOURLS#1861, but I'll take another look at this later to see if there's anything than can be done about it.