tor-gencert should warn nicely when PEM passphrases are too short

If you do $ ./src/tools/tor-gencert --create-identity-key and then give a horribly insecure passphrase like "tor" as the passphrase to the PEM certificate, tor-gencert will give this rather cryptic error message:

Jul 28 18:46:45.709 [err] Couldn't write identity key to ./authority_identity_key
Jul 28 18:46:45.710 [err] crypto error while Writing identity key: problems getting password (in PEM routines:PEM_def_callback)
Jul 28 18:46:45.710 [err] crypto error while Writing identity key: read key (in PEM routines:DO_PK8PKEY)

It would be nice if instead it just said "I require a passphrase with a minimum of 8 characters!" or something like that.