dim: replace message characters leading to decoding errors with U+FFFD

The character set decoding added in commit b66d07db ("dim: decode
email message content charset to unicode") started failing with unicode
decoding failures under certain conditions. (Specifically python 3 and
mboxes downloaded from patchwork.)

Instead of raising UnicodeDecodeErrors, replace values that can't be
converted with U+FFFD (REPLACEMENT CHARACTER, �).

Reported-by: Dave Airlie <airlied@gmail.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>