Make decoding more error-resilient:
- do not decode date or time (just advance) if there is a general error
  (wrong minute length, bit 0/20 wrong, bit 17 and bit 18 equal) as is
  already done for parity errors.
- do not set announce flags if there is general error.

  Note that if an announce bit is erroneously set while there is no
  general error, then the flag is also erroneously set. This might be
  limited by checking the hour value.