Fail fast if we cannot get symmetric encryption key.
Fail fast if we cannot get symmetric encryption key.

For example, if user is not recipient, or doesn't have one's private key
set up right. Otherwise, script would proceed to use undecrypted symmetric
key, efficiently scrambling all information in the repo.

Change-Id: I0e1cd78ea7497cd4642354efe8a64fcfdea7574c

Linaro/git-gpgcrypt