Bad cache defense

This is belt and braces for https://rally1.rallydev.com/#/11744709050d/detail/defect/17407386016

I was able to reproduce the exact error in my development environment by running
CM without memcached in the background. The error occurs for all pages that call
for uploading an image, not just when creating MSBs. I can no longer reproduce the
error on QA or Production.

When this problem occurs, it's because the model has an "image_cache_id" which
means that at some time in the recent past, a user has unsuccessfully submitted
a request and has been asked to fix up some invalid fields. However, the class
CachedFile can't find the id (maybe it's been evicted from the cache or in the case
of my dev environment, the cache wasn't even running).

I'm having trouble creating an acceptance test for it because the default Rails cache
is used for those and everything works great (I suppose I could add a test helper that
evicts the cache entry we're relying on but that seems tricky for not a huge gain).

In any case, a bad cache value should not trigger an exception when checking a
model's validity so this pull request just fixes that.