Fix multiple bugs in EncryptedPassword

Fix multiple bugs in EncryptedPassword

* openssl_decrypt() expects the encrypted string you give it to be the
  exact one that came out of openssl_encrypt(), it doesn't expect you to
  pre-decode the base64 encoding. So don't do that.
* Use the same IV when re-encrypting the underlying hash for comparison.
* Check the return value of OpenSSL functions, and report meaningful
  error messages, for sysadmin convenience and to avoid e.g. giving all
  users the same hash if an invalid cipher method was chosen (which was
  the previous behaviour).
* Fix EncryptedPassword::update(). Tested it with eval.php since there
  doesn't seem to be any callers.

Change-Id: I3a39de152d0329f93d16aa4ed43faf08f665b8e2