File not available errors

Hi,

There are often Freemarker error messages when files go missing (which, they do a lot in our more unstable local developments).  It's better to throw a sensible exception and to get a better error message than to let the error bubble up to a big ugly Freemarker error message.

Compare: 
![old_way](https://f.cloud.github.com/assets/790199/1206822/0a64f6ec-259e-11e3-9ef3-f4a7c88d516b.png)

To: 
![new_way](https://f.cloud.github.com/assets/790199/1206824/10c4a5a0-259e-11e3-9203-cb107e54d8d6.png)

Huw