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/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>