* tool/transcode-tblgen.rb: 8bit byte of ASCII-8BIT is a valid
(but unique to ASCII-8BIT) character.
* transcode.c (rb_eConversionUndefined): new error.
(rb_eInvalidByteSequence): ditto.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@18524 b2dd03c8-39d4-4d8f-98ff-823fe69b080e