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 %>

app/views/static/error.html.erb

@@ -1,10 +1,7 @@
-<% if CurrentUser.user.try(:is_builder?) && @exception.present? %>
-    <h1><%= @exception.class.to_s %> exception raised</h1>
+<h1>Error</h1>
+<p><%= @message %></p>
 
-    <p><%= @exception.message.dup.force_encoding("utf-8") %></p>
-    <%= render "static/backtrace", backtrace: @exception.backtrace %>
-<% elsif @error_message %>
-  <p><%= @error_message %></p>
-<% else %>
-  <p><%= @exception.message.dup.force_encoding("utf-8") %></p>
+<% unless @expected %>
+  <h6>Details</h6>
+  <%= render "static/backtrace", exception: @exception, backtrace: @backtrace %>
 <% end %>

app/views/static/error.js.erb

@@ -1,2 +1,2 @@
-var message = <%= raw @error_message.try(:to_json) || @exception.message.to_json %>;
+var message = <%= raw @message.to_json %>;
 Danbooru.Utility.error(message);

app/views/static/error.json.erb

@@ -1,9 +1,5 @@
-<% if @error_message %>
-{"success": false, "message": <%= raw @error_message.encode("utf-8", {:invalid => :replace, :undef => :replace, :replace => "?"}).to_json %>}
-<% else %>
-  {
-    "success": false,
-    "message": <%= raw @exception.to_s.encode("utf-8", {:invalid => :replace, :undef => :replace, :replace => "?"}).to_json %>,
-    "backtrace": <%= raw Rails.backtrace_cleaner.clean(@exception.backtrace).to_json %>
-  }
-<% end %>
+{
+  "success": false,
+  "message": <%= raw @message.to_json %>,
+  "backtrace": <%= raw @backtrace.to_json %>
+}

app/views/static/error.xml.erb

@@ -1,6 +1,2 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<% if @error_message %>
-<result success="false"><%= @error_message %></result>
-<% else %>
-<result success="false"><%= @exception.to_s %></result>
-<% end %>
+<result success="false"><%= @message %></result>