Rename KeymasterException to KeyStoreException.

The code in question talks to KeyStore which returns error codes
which are a mix of keystore and keymaster error codes. To better
match the layering of KeyStore on top of keystore and keymaster,
this CL renames KeymasterException into KeyStoreException. It also
adds human-readable error messages to exceptions raised by keystore
rather than keymaster (e.g., key not found).

Bug: 18088752
Change-Id: I4cd1235e16518c9f2e8c5557a457774c6e687b88