mailinfo: handle charset conversion errors in the caller

mailinfo: handle charset conversion errors in the caller

Instead of dying in convert_to_utf8(), just report an error and let
the callers handle it.  Between the two callers:

 - decode_header() silently punts when it cannot parse a broken
   RFC2047 encoded text (e.g. when it sees anything other than B or
   Q after it sees "=?<charset>") by jumping to release_return,
   returning the string it successfully parsed out so far, to the
   caller.  A piece of string that convert_to_utf8() cannot handle
   can be treated the same way.

 - handle_commit_msg() doesn't cope with a malformed line well, so
   die there for now.  We'll lift this even higher in later changes
   in this series.

Signed-off-by: Junio C Hamano <gitster@pobox.com>