Get rid of a rescue nil

From the commit (cfdf97d) which introduced it:

-        begin
-          data = ::IO.read(file).split(/^__END__$/)[1]
-        rescue
-          data = nil
-        end
+        app, data =
+          ::IO.read(file).split(/^__END__$/, 2) rescue nil

The rescue block in the cfdf97d^ tree was introduced in b88c0f5
because of #201. I am not 100% sure that rescueing Errno::ENOENT
is enough, though. Please let me know if it is not.

lib/sinatra/base.rb

@@ -680,8 +680,13 @@ def layout(name=:layout, &block)
       # when no file is specified.
       def use_in_file_templates!(file=nil)
         file ||= caller_files.first
-        app, data =
-          ::IO.read(file).split(/^__END__$/, 2) rescue nil
+
+        begin
+          app, data =
+            ::IO.read(file).split(/^__END__$/, 2)
+        rescue Errno::ENOENT
+          app, data = nil
+        end
 
         if data
           data.gsub!(/\r\n/, "\n")