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.

I know what you mean, I had been thinking it maybe didn’t make a ton of sense to have this logic here. Could you elaborate a bit more on your suggestion?  Were you thinking I should instead handle it where the xformserver creates the json error object? That would be [here](https://github.com/dimagi/touchforms/blob/1eea5c6a1ffd9e2d112c2be47fe06c20574e9a6e/touchforms/backend/xformserver.py#L67) or [here](https://github.com/dimagi/touchforms/blob/1eea5c6a1ffd9e2d112c2be47fe06c20574e9a6e/touchforms/backend/xformserver.py#L88) probably.

It also crossed my mind that perhaps it would be more appropriate to put the error display logic later in the javascript, like in the backbone views that are actually putting them in the DOM.