Fix blank error pages when error.messages has no to_str

When an exception whose #messages attribute is not a string is
rescued, Sinatra fails to display the error page due to a
TypeError. The problem is that Ruby's + implementation uses type-strong
coercing (#to_str) instead of type-weak coercision (#to_s). This just
forces to_s to be used, avoiding the problem.

(An example of this type of exception is DataMapper::Errors. It doesn't
respond to #to_str)

lib/sinatra.rb

@@ -1313,7 +1313,7 @@ def load_default_configuration!
                   Params: <pre>#{params.inspect}
             		</div>
           			<div id="stacktrace">
-          				<h1>#{Rack::Utils.escape_html(@error.class.name + ' - ' + @error.message)}</h1>
+          				<h1>#{Rack::Utils.escape_html(@error.class.name + ' - ' + @error.message.to_s)}</h1>
           				<pre><code>#{Rack::Utils.escape_html(@error.backtrace.join("\n"))}</code></pre>
           		</div>
           	</body>