Add a more descriptive error message for messed up cache file error

Addresses this ticket: http://manage.dimagi.com/default.asp?131898

Gives a cleaner error message to the user:

    "Unfortunately an error has occurred on the server and your form cannot be saved.
    Please take note of the questions you have filled out so far, then refresh this page and enter them again.
    If this problem persists, please report an issue."

Note that we don't expect this error to happen very frequently at all.
Yeah, my first instinct would be exactly where the first link was. So basically you would trap `TouchFormsException` and handle it differently - possibly by just sending down a flag that is like `"error_type": "EmptyCacheFileException"`. Then your rendering code can have a `dict` property where it maintains known errors that have already been shown, and if any pop up a second time they can just be skipped.