feat(widgets): do not require email if using Netlify contact form

Netlify routes messages from contact form to its admin panel, so no
email address is required.

By not requiring email in this case, it is not revealed to any spam bots
which might try to scrape a website's public Git repository.

Close #775

layouts/partials/widgets/contact.html

@@ -11,12 +11,15 @@
   <div class="col-12 col-lg-8">
     {{ with $page.Content }}<p>{{ . | markdownify }}</p>{{ end }}
 
-    {{ if and $.Site.Params.email $page.Params.email_form }}
+    {{ if $page.Params.email_form }}
 
     {{ $post_action := "" }}
     {{ if eq $page.Params.email_form 1 }}
       {{ $post_action = "netlify" }}
     {{ else }}
+      {{ if not $.Site.Params.email }}
+        {{ errorf "Please set an email address for the contact form using the `email` parameter in `config.toml`. Otherwise, set `email_form = 0` to disable the contact form." }}
+      {{ end }}
       {{ $post_action = printf "action=\"https://formspree.io/%s\"" $.Site.Params.email }}
     {{end}}