Fix "RuntimeError: can't modify frozen String" in static/error.html.erb.

Fixes a frequently occurring error in New Relic.

app/views/static/error.html.erb

@@ -1,7 +1,7 @@
 <% if CurrentUser.user.is_builder? && @exception.present? %>
   <h1><%= @exception.class.to_s %> exception raised</h1>
   <ul style="font-family: monospace; font-size: 1.2em; list-style-type: none;">
-    <li><%= @exception.message.force_encoding("utf-8") %></li>
+    <li><%= @exception.message.dup.force_encoding("utf-8") %></li>
     <%- Rails.backtrace_cleaner.clean(@exception.backtrace).each do |b| -%>
       <li style="list-style-type: none;"><%= b %></li>
     <%- end -%>
@@ -9,5 +9,5 @@
 <% elsif @error_message %>
   <p><%= @error_message %></p>
 <% else %>
-  <p><%= @exception.message.force_encoding("utf-8") %></p>
+  <p><%= @exception.message.dup.force_encoding("utf-8") %></p>
 <% end %>