application controller: clean up exception handling.

* Simplify code.
* Show backtraces for all users, not just builders.
* Show backtraces only for unexpected server errors (status 5xx), not
  for normal client errors (status 4xx).
* Log expected errors at info level (reduce noise in production logs).

app/views/static/_backtrace.html.erb

@@ -1,6 +1,10 @@
 <%# backtrace %>
 
 <ul class="backtrace">
+  <% if exception.present? %>
+    <li><%= exception.class.to_s %> exception raised</li>
+  <% end %>
+
   <% Rails.backtrace_cleaner.clean(backtrace).each do |b| %>
     <li class="backtrace-line"><%= b %></li>
   <% end %>