irkerd: Handle UnicodeDecodeError in LineProtocol.data_received

irkerd: Handle UnicodeDecodeError in LineProtocol.data_received

I just got the following in a message-of-the-day from
leguin.freenode.net:

  Welcome to leguin.freenode.net in Ume\xe5, Sweden, EU!

Where Ume\xe5 is Ume{U+00E5 LATIN SMALL LETTER A WITH RING ABOVE}.
\xe5 is the ISO-8859-1 encoding.  Since important messages from the
IRC server should be in ASCII [1]:

  Regardless of being an 8-bit protocol, the delimiters and keywords
  are such that protocol is mostly usable from US-ASCII terminal and a
  telnet connection.

So rather than trying some fancy charset-detection heuristics, just
drop lines that don't decode properly.

[1]: http://tools.ietf.org/html/rfc2812#section-2.2