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.
Here is the error I was getting:
```
Traceback (most recent call last):
  File "submodules/touchforms-src/touchforms/backend/xformserver.py", line 6, in <module>
    import xformplayer
  File "/Users/noah/Documents/dimagi/commcare-hq/submodules/touchforms-src/touchforms/backend/xformplayer.py", line 35, in <module>
    import persistence
  File "/Users/noah/Documents/dimagi/commcare-hq/submodules/touchforms-src/touchforms/backend/persistence.py", line 5, in <module>
    from exceptions import TouchFormsException
ImportError: cannot import name TouchFormsException
```
I just tried changing the import statement to `import exceptions` and referencing the class with `exceptions.TouchFormsException`. That raises this exception:
```
AttributeError: type object 'exceptions' has no attribute 'TouchFormsException'
```
No idea why exceptions is a `type` object and not a `module` object though. Some sort of Jython thing I think. See this little test I did:
```
(commcare-hq)Noahs-2014-MacBook-Pro:commcare-hq noah$ python
Python 2.7.5 (default, Mar  9 2014, 22:15:05) 
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import exceptions
>>> exceptions
<module 'exceptions' (built-in)>
>>> quit()
(commcare-hq)Noahs-2014-MacBook-Pro:commcare-hq noah$ jython
Jython 2.5.3 (2.5:c56500f08d34+, Aug 13 2012, 14:48:36) 
[Java HotSpot(TM) 64-Bit Server VM (Oracle Corporation)] on java1.7.0_67
Type "help", "copyright", "credits" or "license" for more information.
>>> import exceptions
>>> type(exceptions)
<type 'type'>
>>> quit()
```