Implement cross-domain script error messages for turtlebits.js.

This allows us to get more detailed error messages from turtlebits.js
(such as CoffeeScript compilation errors) even though user programs
are running on domain user.pencilcode.net and turtlebits.js is
loaded from the (different) bare domain pencilcode.net.
Merge pull request #85 from PencilCode/corserror

Implement cross-domain script error messages for turtlebits.js.