BBufferedDataIO: the underlying stream might return an error.

* In this case, fSize would have been set incorrectly, ignoring the error
  completely, but open the possibility to read from uninitialized or even
  unmapped memory.
BSecureSocket: prepared to do SSL error conversion.

* For now, just print them to stderr; we don't really have matching error
  codes yet AFAICS.
imap: Fetching headers is now working.

* No state is stored yet, though, so that all messages will be downloaded
  over and over again.