Error throwing exception in JRuby

When giving 0.5.0.cr a spin on jruby with windows I ran into an issue where the rendering failed due to unrecognized characters because awestruct was detecting ASCII instead of UTF-8. Anyway I received an error message like this

```
backtrace must be Array of String
org/jruby/RubyException.java:95:in `set_backtrace'
V:/jruby-1.7.3/lib/ruby/gems/shared/gems/awestruct-0.5.0.cr/lib/awestruct/page.rb:149:in `rendered_content'
```
The issue is that the message being passed into StandardError is invalid.

This PR fixes it, but I'm not sure if this is the standardized way we want to be building our exception messages.