kshareddatacache: Add "corrupt cache" exception.

The new exception is not actually thrown yet, but this commit provides
support for handling such an exception, using the following guideline:

* Any exceptions used never cross the API. i.e. client code can be
assured that the underlying library code will not throw any exceptions
when called through the public API. It is simply an implementation
aid.
* If Private::CacheLocker succeeds, it means both d and d->shm are
valid.
* The d-pointer is not necessarily set now, as constructing Private and
mapping shared memory will involve sanity checking the cache as it is
loaded. This affects code using the d-pointer or d->shm without the
CacheLocker.
* Private::CacheLocker itself must still be usable if the d-pointer is
not set, in addition to its existing ability to handle d->shm not
being set.

For just evaluating code changes it might be easiest to use "git diff
-b", which ignores whitespace (there are significant whitespace changes
from adding try{}/catch{} blocks).