added in a 404 image, error handler code and used the corresponding function for thimble's 404 page

fixed the white spacing issues and seprated methods for error handling and nunjucks into their own files

fixed the white spacing issues and seprated methods for error handling and nunjucks into their own files removing the commented out nunjucks code

forgot a new line in the nunjucks file

made errorMessage result in friendly error mesage text

changed white spacing
Merge pull request #257 from elnushaj/bug884351

fixed bug 884351, create a custom 404 page for thimble