Add a template for failed new diary comment validation

[AntonKhorev]

Here's a problem* that I came across when trying to create a controller for diary comments.

You write a comment for a diary entry, submit it and it fails the validation. Currently it's difficult to fail the validation, because empty comments are prevented client-side and it's difficult to type special characters that are disallowed. But let's suppose the validation failed. We have a branch for it:

openstreetmap-website/app/controllers/diary_entries_controller.rb

Line 154 in 50df5ee

render :action => "show"

It just renders the diary entry page again. You'll be looking at the top of the page, not where the comment form is. And the entered comment is reset:

openstreetmap-website/app/views/diary_entries/show.html.erb

Line 37 in 50df5ee

<%= bootstrap_form_for @entry.comments.new, :url => { :action => "comment" } do |f| %>

Here I'm showing a different page instead. It is similar to subscribe/unsubscribe pages and has a comment form that keeps the previously entered comment.

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* The actual problem was that with a diary comments controller I'd have to render the show action of another controller.