Don't try to be too clever on failure in UnsealFromTPM

[chrisccoulson]

Currently, UnsealFromTPM tries to determine between an invalid key file or a TPM provisioning issue, and executePolicySession goes even further - trying to determine whether the static metadata or PCR policy metadata is invalid, although that isn't exposed at the moment. I think this should just return a single error on unseal failure (InvalidKeyFile), and then it should be up to the running system to perform additional diagnostics (see #33)